When I'm declaring blockers (or attackers) do I have time to activate abilities such as regenerate before damage is assigned?
Yes. You can cast a spell after blocks are declared but before damage. Otherwise {Chaotic Strike} wouldn't be very useful.
Attack Phase goes like this.
Beginning of Attack Phase
Announcing Attackers
Announcing Blockers
Damage Resolves
End of Attack Phase
So you can say, tap things in response to end of the Beginning of Attack step or cast pumps at the end of the Block step. Lots of chances to do stuff.