I tend to think of hair that is multi-colored. It doesn't necessarily need to be black though. I do think elaborate hair is typically gothic-styled as well.
I agree, there are so many different types of hairstyles that could classify as gothic that it really is such a wide range. I don't think that there is an exact definition.
Wow, I have not seen a crimping iron in a while now, and I haven't seen many people with crimped hair lately. It will be interesting to see what new styles emerge with them next.