Can I remove blocking to level floor and sister this joist?
Yes, you can remove the blocking. Blocking is NOT structural. 

Blocking is only used to keep structural members plumb so they don’t lay over to one side when “fully” loaded. 

Actually, blocking is not required until the structural member gets to a ratio of 6:1 (i.e.: 2x12, etc.) (See IRC R502.7.1)