I just went from black to blonde not too long ago (altho I'm now a medium auburn). What I did to go lighter was buy a color remover (I think there is one called "Oops", I'm not sure which one I bought) and take the black out of my naturally dark brown hair, then I bleached it (twice). It was then light enough that I could have dyed it just about any color, even a different blonde color. Granted my hair was a bit fried, but a twice weekly deep conditioner and some leave in conditioner on the other days I wash my hair seems to have done the trick. Plus the more dry and fried your hair is the better it soaks up the funky colors (in my experience). I think turquoise would look really nice on you. Manic panic makes one called Atomic Turquoise, tho I think that might be a bit lighter than what you're looking for.
~*~*~Don't let today's moment forsake tomorrow's dream~*~*~