I don't have a racing horse, but my horse has an issue with mounting blocks.
The way we've deal with it is he stands (ground tie now.. but before I held his rope) and I got to pick the mounting block up, and put it down. Up then down. Stand on it. Get off it.
If he moved, I'd undo his movements. If he backed 2 steps, I'd bring him forward 2 steps.
Then I'd get on the mounting block and if he didn't flinch I'd stroke his neck and ruffle his ears and do everything he loved on that block. When I got off it, the loving stopped. Back on it, love on him, off it and it stopped.
Then when he was okay with that, I'd move the block around him. First I'd drag it and he'd spook away but I held him. Made him come back and stand. Then I'd drag it again and would stop before he blew up. I'd rub his neck as a praise and give him a few seconds to breathe. Then I'd drag it again, and if he didn't flinch I'd love on him and stand on the block.
I'd scuff my feet on the block, not paying any direct attention to him. If he stood there I'd praise him.. if he didn't, get off the block, undo his steps, back on the block and scuff it. If he didn't flinch, I'd praise him and get off the block.
So now he was fine with me moving it, dropping it, scuffing sounds on it, standing on it, etc. Now the final test was adding the saddle and mounting in.
Now when you mount, you have to be careful not to jab your toe into their side. That causes them not to like the block, or being mounted. Stand on the block and push one hand into the stirrup like your foot would be. If your mare moves, get off, put her back and get on the block again. Same thing, add pressure on the stirrup if she doesn't move, remove pressure and praise her. Then grab the saddle and wiggle it around. If she moves, get down and put her back.. back on the block and do it again. Praise her when she stands.
Then comes mounting. It helps to have a friend hold your off stirrup and horse for you. By now, your horse should be alright with being mounted. Just be sure not to indirectly ask for your horse to move via kicking or squeezing.
I know this is very drawn out but again.. mounting block issues can get you hurt. I had a friend whose horse shied from a block when he was nearly on (and it caught the horse's eye).. flew sideways and he landed tailbone first on the block.. couldn't sit comfortably for a good 2 weeks.
Now if this fear extends to under saddle as well.. then you need to do some ground work and riding involving the block. I have it so my horse now stands next to it and can trot by it without jumping 6 feet sideways.
Best of luck :)
"Strength is the ability to use a muscle without tension"