I miss Ken's ultra alot too. the only real way to get it consistantly is when theyre coming out you with the high jump kick or if you can get em with a focus. otherwise its just anti-air which isnt too bad. but some people its harder to land it on, chun li being the main one.
and as far as ken vs blanka it can be frustrating but for me, i typically lose to them when im too aggressive. with blanka i just sit back and hadouken until he wants to come at me with some sort of flip across the screen move then you just punish with either an ultra or shoryu.
Ken can land his ultra like Ryu can - I recommend using Fierce SRK -> FADC -> Ultra. If you time it right you can get all parts of it to connect.
Also Super -> Ultra works, but it's pretty weak and I only use it for the kill.
Blanka definitely is a frustrating match up. He is a difficult match-up for all shotos. I try to bait the Blanka Ball by switching between throwing Hadokens and randomly hitting jabs, sometimes they'll go for it and eat a Shoryuken.