For pitchers I really don't have something, but for hitters I have found a great product.
My son (not a pitcher) used to get blisters on his hands all the time during batting practices where about 400 hit balls take place. He started using something called Body Glide on his hands during BP and did not have any more blisters. We passed the word around the training place and all kids with blister problems started using it with same great results. The Body Glide is used by runners, but works great for players during BP. Simply rub the hands with it before putting the batting gloves, and repeat as necessary. I even called the company and told them about the success we have seen and they did mention that many people have called about it for baseball. The company site is at www.bodyglide.com if you want to check it out.
If a blister already happened obviously there is nothing to do, but in the past I have applied Vicks VapoRub and it helps heal a lot faster and dry the blister. Maybe a pitcher can rub his hands with it before throwing and since there is no residue, I believe that it would be legal and maybe it could help prevent blisters, but I have not tried that application of the product.