How does removing +1/+1 counters during combat affect abilities granted from having that counter?
Once blockers are assigned, you do not recheck legality of assignments again. The creature is considered blocked and combat damage is exchanged between them normally. The same would be true if an attacking creature became unblockable after a blocker was assigned to it. It would still be blocked.

From the Comprehensive Rulebook:


  509.1g Each chosen creature still controlled by the defending player becomes a blocking creature. Each one is blocking the attacking
  creatures chosen for it. It remains a blocking creature until it’s
  removed from combat or the combat phase ends, whichever comes first.
  See rule 506.4.