Developer volumes

I currently have dyed black hair, naturally dark brown.  I am planning to use color b4.

After that, I'm going to bleach my hair, I don't know how light yet.  The purpose is to lighten it, so I can dye it very blue black and get a good hue.

Since I'm going to be bleaching, I thought I would have some fun and go really blonde for a week (reccomend a good toner/blonde color?), and then maybe use some of the blue dye for blue hair another week before I go blue black.
(FYI I'm using Sally's Ion Color Brilliance perm. black, and the for the blue their "brights" line"

I have never bleached my hair before so I'm trying to understand volumes.
Should I:
A. Use a 20 volume and leave on until completely bleached? possibly a long time?
B. Use a 40 volume and bleach faster?
C. Use a 20 volume and do 2 or 3 sessions

I don't want to I plan to use the coconut oil method.

Also, what bleach would you recommend?  I was just going to use Sally's "Salon Care" brand for the developers (unless there is a better one), but haven't found a bleaching/lighter product yet.
I was looking at L'Oreal Super Blue Cream Oil Lighter ( http://www.sallybeauty.com/creme-lightener/SBS-500509,default,pd.html#q=oil+lightener&start=5 )
and the Clairol Blonding Kit/7th Stage ( http://www.sallybeauty.com/clairol-blonding-kit/SBS-320815,default,pd.html#q=oil+lightener&start=1 )