For Speedy Blood Frenzy, I'd consider making it permanant, and lowering the damage bonus, because feats aren't something people should have to keep track of to remember if they were used in the encounter. Powers already fill that role. Also, think about making the bonus apply if the enemy couldn't see you are the start of your turn. That way, you get the whole "run out and stab before they have a chance to react" thing going on.
Something like this: You get a +2 damage bonus to enemies that couldn't see you at the start of your turn. Increase the bonus to +4 at level 21.