Gumpa,
It could be that the fly ate something that doesn't sit right with your trap, that a hole or some type of damage was on the trap, and that once closed, the right conditions for a bacaterial assualt on it existed.
Traps do die and turn black, perhaps these traps had reached the end of the road.
You plant does not need to eat to survive. I would suggest holding off the feeding for a few weeks, maybe until next month, and let's see what happens, let your plant recover, and then maybe try feeding it a fly one more time.