This is mostly an academic question, but here it goes.
I have hair of the darkest brown (easily mistaken for black, but not technically black.) One time my then-girlfriend wanted to dye my hair blue so, of course, she bleached it first, and then applied blue dye. The thing was that, over time, it just turned green.
Were I to ever want to dye my hair blue again, and I'm not sure I'll ever be in a situation where I CAN but I'm still curious, what would I want to do differently, or is the green fade inevitable? Was the issue withthe quality of the dye? If I don't bleach first will the blue show up at all? I kind of like the idea of keeping my dark hair, and just adding a blue tint to it, but I know nothing about thisstuff, and don't know if the hair needs to be bleached for the dye to be absorbed at all, or what.
For your trouble, I'll find a picture of myself with blue and/or green hair later, and post it for your amusement.