As far as I know, the witch is the only way to get black slimes. I keep a wicked statue in there because I don't want all my slimes turned black, especially the tiger slime. I'm not sure about the brown/copper slimes either, but I'm betting they can be gotten through breeding. I did notice that my slimes bred some grey ones. I hate going in there to clear out green slimes because it's hard to pin down one slime at a time. I certainly don't want to accidentally kill any of the red, purple, or tiger slimes. Maybe I'll build myself a pen to breed some slimes outside and see if they breed any brown or black ones.