Blanka is probably the easiest character to get immediate results with.
Beginners will struggle to punish Blanka for any of his moves and really struggle to stop his Ultra, so you'll find yourself picking up cheap and easy wins for relatively little effort.
Once you come up against players who can stop those attacks, however, you'll be forced to dig deeper and reach into Blanka's box of tricks to see what you can pull off...
Rolling Attack (charge back, towards + P)
Blanka's day one go to move, that almost everyone will be abusing. You can combo into it with enough practice (jump in HK, cr.LK, st.LP xx Rolling Attack) but most players will just randomly throw them out. Because it's so hard to punish and stop, it seems like a good tactic.
But be careful, as EVERY character in the game can retaliate. Some need an Ultra (like Abel) while some don't (like Dhalsim), so experiment and see what you can get away with. Also, EX Rolling Attacks pass through fireballs. This is very hard for shutting down a projectile game. Another way of passing fireballs is sliding under them with his down/towards + HP.
Backstep Roll (charge back, towards + K)
A fairly useless move. To slow to react to fireballs with, the only advantage about this move is that it hits overhead (so has to be blocked high) and can cross-up easily (so it can be hard to guess which way to block) but experienced opponents will deal with this move no problem, so don't get into the habit of using it.
Vertical Roll (charge down, up + K)
This is Blanka's anti-air special move. As long as you have it charged, it will be hard for opponents to jump at you. Just knock them out of the sky with this move. Be careful they don't jump past the arc of this move, as Blanka jumps up at a certain angle. You want to make sure it hits when you go for it, as it leaves Blanka wide open if he misses.
Electric Thunder (mash P)
Press five punch buttons will get this move to come out, so drum across the punch buttons twice (if played on an arcade stick, of course) to always get the move to come out.
It's a decent move, for chip damage, as a wake-up option, as an anti-air... but it does have weaknesses. You can be hit low out of it, it's still suspect to projectiles and far reaching command grabs will still grab Blanka right out of it!
Ground Shave Roll (charge back, towards, back, towards + P)
Blanka's super that sees him move up into the air, then roll towards his opponent. Good damage if you spot an opening but it's better to save your meter for EX Rolling Attacks.
Lighting Cannonball (charge back, towards, back, towards + PPP)
This is the Ultra that will annoy everyone. It's really hard to block when performed at point blank range, as it either hits low or hits as an overhead (which must be blocked high) depending on just how close you are. So not only does the opponent have to guess which way to block but it does an incredible amount of chip damage even if he does block correctly.
This is Blanka's ultimate comeback tool. If the opponent does block correctly and survive, the recovery means Blanka is in big trouble but the sheer weirdness of not knowing which way to block means it's your ticket to winning a match you don't deserve to.
Basic Combo: Jump in HK, cr.HK
Advanced Combo: Jump in HK, cr.LK, st.LP xx Rolling Attack