actually, black ich is not true ich. and it is not nearly as bad to deal with either, "black ich" can be easily defeated with formalyn treatments.
I have heard one theory that ich exists in all fish, and that it breaks out due to stress, etc. either way, it has a life cycle, and when mature, will drop off the host, into the tank, where it will become free seeming and infect all fish in your tank. I've only dealt with ich once (knock on wood), and it is sad to watch a healthy fish succumb to it. it can be treated with copper (not in the main tank) and an even better method is hyposalinity (also not in the main tank). Beth has a great thread on ich and hyposalinity in the disease forum.