I dunno if people just can't use him or not, but as far as I can tell Blanka could use a few more moves, he has his rolling attacks and that's about it. I've only played him quickly in arcade so I can't say for sure but that's all he seems to do.
I'm currently expanding my character roster, I'm kinda bored of playing the women all the time so I'm trying out Fei Long, Dhalsim and Gen.
On a side note I swear I'm destined to fight Sagat when I use Sakura, every time I pick Sakura before my opponent picks their character they end up playing Sagat, it's happened too many times to be coincidence!