HI! I actually just posted a video of myself touching up my roots and going pastel blue.
it can probably help answer a lot of questions you have about products i use and what i mix for the bleach part.
http://www.youtube.com/watch?v=kxSTQ2jFvMc&feature=share&list=UU9PmpL3BGa6IGg54Hp1FZKQ
for my dark hair i usually have to do it about 3x plus using purple shampoo to tone it into a platinum/white blonde