My guess is that it has to do with some type of drug use, likely heroin [like "Black" tar heroin...also seen in the GooGoo Dolls song Black Balloon]
besides the name, the fact that he says things like "Call my name through the dream and I'll hear you scream again" and he refers to the Black Hole Sun as coming to 'wash away the rain' which could be interpreted as his doing drugs to take away the bad times...
