From the longer Wikipedia page 
John Blount, 3rd Baron Mountjoy (c. 1450 – 12 October 1485) was an English peer and soldier.
In 1484 James Blount became disaffected from Richard III, and a supporter of Henry of Richmond, the future King Henry VII. This became apparent to King Richard when, later that year, he ordered the return to England of John de Vere, 13th Earl of Oxford, who had been held prisoner at Hammes for nearly a decade. Thomas Montgomery, who was deputizing for Mountjoy, also went over to Henry's side. James Blount had taken Oxford to Henry's court, and in December an attack was mounted on Hammes by John Dynham, 1st Baron Dynham, captain of Calais. In January 1485 Oxford, with Thomas Brandon, successfully evacuated the force from Hammes, including Blount's wife, and his brother, James Blount, and they joined Henry. Richard, seeing the affinity of William Hastings, 1st Baron Hastings (died 1483) falling away, negotiated inconsistently, with offers of pardons mixed with confiscations, and John of Gloucester was brought in over Dynham. James Blount and others were with Henry when he invaded England and became King after defeating Richard III at the Battle of Bosworth.