I have had the merena with no period for 2 years now. about a week ago i had one bad cramp. and started bleeding black blood and black clots. why?

A period?: It's very common for people to lose their periods while on the mirena (levonorgestrel). The lining of uterus shrinks and there isn't much to come out. But once in a while people do get a period. It could be because the iud hormone level is drifting down over time, or it could just be random. It's less blood than normal, and the blood can be dark. I wouldn't worry. Wait and see what happens next month.