black battery cable terminal and battery post...why?

A few days ago, I noticed my positive battery cable terminal was black and it appears that the positive post is the same way. The battery went dead, and I charged it. The car had a hard time starting, but since it started and I wanted to drive the car to work...off I went. Of course, this morning leaving work (3rd shift), the car wouldn't start. Had a guy try to jump start it and still nothing. Pulled the battery and put it on the charger at work and the voltage showed good, put it back in the car and now the voltmeter won't even move and the car won't even attempt to do anything.

I bet if I put a different battery and positive cable on it, everything will be fine, but my question is what caused this. Could it be a dead cell in my battery? Think it's just a freak occurance? I think the problem is more so a bad connection because of the black(burnt) positive battery cable terminal, but I just want to know what caused it to become black(burnt) in the first place.

Any ideas?

Thanks

hmm...I asked my bro if he knew what the problem was and he said he's never seen or heard of it before and neither have I. He said I probably should take the battery to get it tested and ask the people at autozone what causes that before I put a different ($120 optima from the GT) in there and ruin it. Sounds like the best course of action to me because I really don't want to ruin the optima.