Pretty much, you cant dye over black hair and even with the dark brown the purple is not going to show. Your best be would be to go to a professional because your hair either needs to be bleached or you need the color stripped out of your hair first, both use very strong chemicals and doing it yourself isnt smart and you will most likely fry your hair. Also bright colors such as purple and pink are very hard to get even coloring through out your hair when you dont know what youe doing and you end up with blotches and streaks.
it depends on how dark purple you want it. since your hair is black, and you dyed it purple, it would probably turn out to be black with a purple tint to it. but if you dye it blonde first, you might want to wait a while in between the blond and the purple, since dying isnt very good for your hair. (: i would reccomend going to a professional to get it done. (:
They have purple, blue, and red hair dye at Wal-Mart. Also, I was only going to dye a section of my hair purple. Like my bangs, which are parted to the side, and the rest of my bangs left on the other side. My always dye my own hair, I dye it black whenever my natural dark brown grows back out. I have also put streaks in my hair at home.
my friend had dark brown/black hair and he died his hair purple and it turned out pretty good. he used XXL Live :)
You need to bleach it until it turns a lighter shade of brown. that way the purple with take to your hair. If you don't it won't show up at all.
the purple dye would have to be a dark color