Hash turned black, wasnt as potent?

Hey blades, quick question. So the other day I made my first ever batch of QWISO hash that was a huge success. What I'm wondering now, is I kept the little ball in a plastic bag. I smoked half of it the first night I had it and I got BLAZED as fuck. I was damn high. However, last night (the night after) I looked into the back and the ball of hash turned almost black, and was pretty close to being rock hard. I smoked it, and it was still dank but nothing like the prior hit. My method of smoking it was placing the ball on a screen in a pipe, and holding a torch light above it until it would boil. I never touched the hash itself with the flame.

So why did the hash turn black, and why was it not as potent the second go around? Was it oxidized if that's possible? How should I keep my hash fresh? I guess a plastic bag wont do lol.

Thanks guys!