It depends on how strong the purple dye is and how black the current black is.

I would say the most likely is that you will not see the purple at all.

Where did you buy the purple and black dye? Is it from Hot Topic or a pharmacy, or is it from a specialty store for colorists, like Sally Beauty Supply? Hot Topic dyes are notoriously weak.

I haven’t dyed my hair black yet.

I love the idea, but check with a colorist before you do it.

I also haven’t bought hair dye yet.

Here are 15 suggestions for dying your hair black with a highlight of a different color.

http://www.fluther.com/147047/how-do-i-dye-my-hair/

I don’t think it will work.

I’ve dyed my hair violet (dark purple) before and I loved it. I suggest you do that.

I can’t find dark purple, where can I find it? Also, violet isn’t dark purple.

@gailcalled, that link goes to my other question. Well done.

Here is a semi permanant dye in dark purple, however, if your hair is a light color it will not go on dark purple. if you have light hair I would suggest darkening it to a deep brown before trying for the dark purple.

I would say not much would happen except you are probably killing your hair.