ESutman
Posts: 10
Location: Massachusetts, US
Current Hair: Dark Blue
If you go to a hair supply or Sally Beauty, you can get a 40 volume developer that will help bring your hair to a nice platinum. 40 vol is the best you can get without a license, but don't leave it on any longer than necessary, because it's crazy rough on scalps and roots.
MaryJane is also right that if you add a gold correcting toner, it will help to cancel out residual yellows (by adding purple/blue) and give you a better color to start dying from.
Personally the best light blue I remember getting came from thinning out a dark blue with conditioner like BattleSheep suggested. The color I usually use is After Midnight Blue from Manic Panic, but you might prefer to go with SFX.
Also, if you don't want yellow tones, lean towards more purple blues.
Lastly, separate out the two parts of hair so that you don't have to dye black onto bleached hair. Black is such a harsh color chemically, you might well lose it if you start with an intense bleaching, and besides, it'll use a lot less bleach..