i wud be using the larger part of there mixtone, we use tigi colour to but aint had request for blue black as yet. if it were me i wud be using 4/1 and then puttin panels of 1/0 mixed with a hella lot of 0/11 through it. blue is a pretty volatile colour to try and keep in the hair so chances are it will give a blue hue for a while and will eventually fade out to blackish!!
i got a radiant gloss concoction from our rep that was called dark heather and it was 4/0 with 5in 0/7 green ash tone and 2.5in 0/11 blue tone and it came out real dark and my lil sis said it looked blue black, so u cud try 4/0 with jus a hella lot of 0/11? sorry if that aint much help jus chuckin a few suggestions in xx