@Mor - The only experience I've ever had with anything that could be called "black bile" was when I was vomiting what looked like black bile.
I think that was because I was very sick, but I'm still not sure why that happened. I've read since then that it was probably blood, because bile is usually yellow or brown.
I imagine what they thought of as black bile in the four humours theory was actually various different substances that happened to turn dark. Maybe even bloody stools.
Or maybe they didn't think black bile came out of the body very often. It seems to be associated with internal conditions like depression, after all.