Blanka’s low jump can be DP’d on reaction. You have to be semi-good at doing that actually if you want to be good at that matchup I think, since Blanka’s low fierce and slide will beat you if you’re randomly sticking out early far s.HP’s to try and discourage low jumps. Up close, then yeah, it’s hard. But if you’re at around half-screen distance (where you can consistently react to slides), there’s no reason you can’t react to low jumps as well.
Don’t do RC uppercut as anti-air. You’ll still lose to Kim’s anti-anti-air super so it’s pointless. TIGER UPPERCUT!!! (mash the buttons in rhythm for no reason) is already one of, if not the best anti-air attacks in the game by itself already.