I had dark brown hair with some blonde highlights put through the underneath (the clever way so you don't have roots). . anyway I decided to go blue/black. Well, where I'd had the blonde, after a few weeks, it faded to green so it was a nightmare as I had to have it dyed all the time! I now have it a colour called red/black which looks black at night but quite red in the day. All of my blonde is out now which is a relief! But if I were you, I wouldn't go blue/black because you'll be asking for trouble. I'm not sure about the plum though. Good luck