I have dyed black hair!

I've currently got Directions Alpine Green on a section of my hair and my hair is currently dyed dark brown, it does show up quite well and is a really nice colour. I'm not sure if it would show over black, but if the black is a little faded it might? It's probably worth a try before you resort to any other options! (maybe give your hair a few washes with a harsh shampoo like head and shoulders or a clarifying shampoo first, I find that fades even permanent dyes a little).

I will add though, before using Alpine Green be sure that you want green for a while, it lasts ages on my hair without fading!

You could try a strand test first to see if the green will show over the black - if not you could do a colour remover (NOT stripper) they're not damaging as such but are drying so be sure to follow it up with a really good hydrating conditioner.

Come to think of it, if you get a decent result from the colour remover you could slap the green dye on straight afterward.  Direct dyes are like conditioner with a LOT of pigment in  😉