Blight against +1/+1 counters is broken. When blighting, a player is supposed to add -1/-1 counters. If the player selects a creature with -+1/+1 counters, the counters end up not existing, even though there should be both -1/-1 AND +1/+1.

This causes headaches if you go to proliferate the -1/-1 counter, because it suddenly doesn't' exist.

This extends all the way to ANY interaction which adds or removes -1/-1 counters.