Black hair to blonde balayage

So I've been experimenting with my step daughter's hair, she had dyed it demi perm black for about a year, I done a bleach bath as colour b4 done nothing and it lifted a little, few months later I bleached it and it went patchy but lifted quite a bit of the black out, it's now a month on and I 'd like to try a balayage with the bleach, her current colour is 3 inches of strawberry blonde roots, a band of dark brown/black in the mid length then light brown everywhere else, I'm on a budget and have ordered Matrix socolour in 7A, my thinking is to balayage bleach, then out the 7a with 10 vol on her roots and the same 7a but with 20 vol on the lengths, will this work?