So you've learned the hard way. (Join the club!) :)
I colored my own hair a bunch during college and ended up with black hair (naturally a medium brown). I was hired to teach at my first school and realized I would like to look a little more professional than the poor college student. I found a wonderful sylist that highlighted most of my head with two colors. She picked a dark blonde and a medium brown and would pull a set of hair (by interweaving her comb tip through so it wasn't so stripy) and would do one color, the next, and then leave the black. She did this over and over all over my head. My overall affect was pretty cool. The next time she colored it, she did the same thing and over time we went a little lighter and the black disappeared.
I'd tried to have it all stripped before and recolored and that was HORRIBLE. It really ruined my hair. I highly recommend going somewhere reasonable and having them try the highlight approach.
I now do highlights to hide my "natural" highlights. :)
It really pays to find a good stylist at a budget place. I see the manager at the Fantastic Sams here in town and am very pleased with her work. Not as good as the gal I saw when I lived in Vegas, but I'll never find another one of her in my budget. GOOD LUCK! DON'T DO IT YOURSELF!