Yea, I had my friend do it, she has her license and everything. And I left it on there for a few hours, my hair is just pretty much black almost and it's quite stubborn. Not a very good combo, ha.
Try bleaching out the section first, then applying a ash/pearl toner. That will make it so the blue only needs to sit on your hair for about 30 minutes each time, AND it will get rid of those greens that make that turquoise color you didn't want
Check this out: [link]
Thats the blue I did on my friends hair :3
hope that helps!!