The answer can be found in Chapter Fifteen of this excellent piece of non-fiction. Bert Blume was the reason for Pollard's first terrible accident. Pollard and Blume were old friends from long ago, and in this chapter we are told that Blume is in trouble because he needs a rider for a horse. Note the situation that is described:
Blume was in trouble. A rider had promised to gallop a green two-year-old named Modern Youth for him but hadn't shown up. there were no other riders available. The workout was critical; the colt was scheduled to race and needed a blowout. The race in question was a forgettable weekday event for a trifling purse, but Blume was strapped for money.
Pollard was always somebody who would help out his friends if they were in need, and so he said that he would ride the horse for his friend. The accident that occurred made Blume fall into a fit of guilt and go on a "bender." Thus Bert Blume is a minor character who only appears in two pages of this book, but whose impact on Pollard and Seabiscuit is massive in the way that it was thanks to Bert Blume and Pollard's generosity that Pollard was injured so greviously.