This is no longer a matter of genealogy, which is now settled, but simply a question of heraldry. But that does not mean we may just guess and no longer look for evidence. I am very grateful for an article by a certain Todd Farmerie and Nathaniel Taylor, 'Notes on the ancestry of Sancha de Ayala', NEHGR 103 (1998), 36-48, available at http://www.nltaylor.net/pdfs/a_SanchaNotes.pdf, which saved me a great deal of time in my research.
Firstly, we need to look carefully at what was actually happening with quartered arms at the time. Eleanor of Castile had used her father's quartered arms, so the idea was not new, but in 1340 Edward III was the first Englishman to use quartered arms, to represent his claim to France and England. Laurence de Hastings quartered Hastings and Valence in 1345 after he inherited the Valence lands and the earldom of Pembroke. Richard 'Copped Hat' of Arundel quartered FitzAlan and Warenne in 1353 for his two earldoms of Arundel and Surrey. Clearly, quartering originated as a method of showing that one man held two titles and two lots of land.
In 1386 Richard II made Robert de Vere KG Duke of Ireland for life, and allowed him to quarter 'azure, three crowns or, a border argent' for as long as he was lord of Ireland. Later John Talbot KG adopted a new quarter ('azure, a lion rampant and border or') to represent his earldom of Shrewsbury to go with quarters for lands he held at Eccleswall and Castle Goodrich (Talbot, father), Blackmere (Strange, mother), and Hallamshire (Furnival, first wife's mother). William le Scrope KG, Earl of Wiltshire, quartered the arms of Man after he bought that lordship in 1392. Then John de Beaumont KG of Folkingham (d.1396) quartered Beaumont and Comyn of Badenoch, the latter representing his great-grandfather's earldom of Buchan, even though the family had lost the lands that went with the earldom.
Later on, Richard Neville KG, Earl of Warwick and Salisbury, the 'King Maker', used a seal in 1465 with grand quarters to show how he held lands from the Beauchamp, Clare, Warwick, Despenser, Montague, Monthermer, and Neville of Salisbury families. However, his Garter stall plate chooses just Montague and Monthermer quartering Neville of Salisbury. When the head of the Order, Edward IV, married Elizabeth Woodville in 1464 he wanted to make her look less gentry and more nobility. He therefore gave her five extra quarters derived from the ancestors of her mother, Jacquetta of Luxemburg, and relegated the Woodville quarter to last place. None of the quarters represented any land and were simply an unsuccessful attempt to boost her social standing amongst the aristocracy at court. Heraldry had now lost its original purpose of personal identification and become just an empty status symbol.
We now need to look at the arms that were being used by the Blount family. Sir William le Blount of Sodington (d.1337), who as husband of Margery de Verdon was summoned to Parliament, bore 'barry nebuly or and sable'. His heir and brother, John (d.1358), bore the same arms. John's son Walter (d.1403) is the one who married Sancha de Ayala. I am unable to find any direct evidence of arms for him or for Sancha herself.
Sancha's eldest son, Sir John Blount KG (d.1418), appears to have been the first in the family to bear quartered arms: 1 and 4, or, a tower azure (Toledo), 2 and 3, barry nebuly or and sable (Blount). His brother and heir, Thomas, bore the same arms. And Thomas's son and heir, Sir Walter Blount KG, Lord Mountjoy, also appears to have borne the same arms. There is a group of mainly French rolls of arms from around the middle of the fifteenth century that Steen Clemmensen has shown copied, either from each other or from a common source, a very similar section of English arms. This Golden Fleece Group of Armorials includes painted shields for 'le sire de Blont'. One problem is that although the rolls were compiled during Walter Blount's lifetime, they may have been copying information from John Blount's lifetime. Clemmensen chose to allocate the shields to Walter Blount. The next problem is that all the shields are painted 'quarterly: 1 and 4, or, a tower azure, 2 and 3, vairy or and sable'. However, although modern vair is drawn with angular-shaped skins, at this period it was drawn with very rounded shapes, almost the same as nebuly. It only required the original source to mistake 'barry nebuly or and sable' for 'vairy or and sable', and all the other rolls would have copied it.
We then come to the Garter stall plate. The original statutes of the Order have been lost, but they were revised in 1421 under Henry V and at that time included a provision that, on the death of a knight, a plate of his arms and helm were to be placed as a memorial in his stall; those of later knights were not to be larger than those of the founder knights. It was only under Henry VIII that the statutes were amended to require knights to put up their own stall plates within a year of election. Despite this, St John Hope consistently estimates the dates of all stall plates from after 1421  to be around the date of the knights' election, rather than their death. It is of course very possible that a knight might prepare the memorial in his stall at Windsor during his lifetime, in the same way as he might prepare his tomb. But there remains the distinct possibility that the stall plate was in fact arranged by someone else.
Sir Walter Blount's stall plate must date from between 1472, when he was elected, and shortly after 1474, when he died. It is rectangular with vertical stripes of silver, red and translucent green. This is reminiscent of a banner. Indeed, St John Hope points out that most of the plates from 1421 to 1466 are engraved round the edge with a fringed or similar border, like a banner-at-arms; and three of them actually have a staff on one side. He also considers the plate probably to be of foreign workmanship, and notes that what had originally been blue enamel was now a pale brown colour. This suggests it was also of poor quality, since older plates such as those of Guy de Bryen or Walter Paveley (both c.1421) have kept their deep blue colour without any problem.
Across the bottom of the plate is a scroll inscribed in black letter 'Walter Blount s'or de montjoye'. Most of the names and titles on other plates are in French, the language of chivalry, although a few are English. Latin wasn't used until Tudor times. Since the plate is probably foreign, I leave it to others to decide whether Walter's title of 's'or de montjoye' could be mediaeval Castilian or not.
The arms on the plate are quarterly: 1, Ayala (for Sancha's mother), 2, Toledo (for her father), 3, Blount, 4, the notorious 'Vair'. Apart from Blount, none of the quarters represented land. Peter le Neve's Book of c.1399-1500, at PLN 159, has a different version of the Ayala quarter. The stall plate has it as a silver field (and two black wolves) with a silver border (sprinkled with red x-crosses), an unusual combination, whilst PLN 159 has the field silver and the border gold, a much more likely arrangement. Both the stall plate and PLN agree that the 'Vair' quarter is in the normal combination of silver and blue.
We now come to the crunch: what is the significance of the vair quarter? The real answer is that we don't know because we don't have enough evidence. Based on what evidence we have, there are a couple of possibilities: the Blounts decided they wanted to show descent from Beauchamp of Hatch (whether true or not), or Sir Walter wanted to add a symbol for his new lordship of Mountjoy. Whichever one, I don't want to take sides because I don't see the value of just guessing.
 Cooke's Ordinary (c.1340) CKO 512, Cotgrave's Ordinary (c.1340) CG 430.
 seal: 1343, Birch 7507.
 Armorial Anglais (c.1400) ARS 92; Armorial de la Ruelle (c.1400) 233 r6; Bruges's Garter Book (c.1430) BB 111 (stall P4) (DBA i. 96, ii. 246).
 seal: 1427, G Demay, 'Inventaire des sceaux de la Collection Clairambault' 1102; Peter le Neve's Book (c.1399-1500) PLN 1904
 S Clemmensen, 'The English in the Golden Fleece Group of Armorials', The Coat of Arms, 3rd ser., vol II part 1 (Spring 2006) pp 11-44.
 ibid., p 36.
 Armorial équestre de la Toison d'or 798, Bergshammar Armorial 1990, Armorial Clémery 429, Lycenich Armorial 654, Nicholas de Lutzelbourg Armorial 162, Armorial of the Peace of Arras 292.
 W H St John Hope, 'The Stall Plates of the Knights of the Order of the Garter 1348-1485' (1901) pp 7-9.
 ibid., p 9.
 He produces very good arguments to show that all the stall plates now extant for those knights elected before 1421 were produced at the same time, namely shortly after that date.
 ibid., p 16.