I dont mean beat it outright but punish a blocked one. Many people will get hit by random Blanka balls in which is not that hard to avoid if you know they are charging. If you mean pre-emptively know when its coming to stop you just have to know when they are charging.
I just read up some strats and they said the same thing, punish a blocked ball with EX/MP/FP Seismo. If its a vertical ball then it has to be EX Seismo, and Rainbow Balls are Focus Attacks.
Again i don't think Viper has anything that outright beats a Blanka ball while its in motion. Punishing one after a block is easy though.