Just going by the genes alone, The doe with a blue carrier has one dilute gene and one false-dilute gene same with the black buck. The blue buck has a pair of dilute genes and no false-dilute. Only rabbits with a pair of dilute gene will be blue.
So your black doe X black buck will on average give %25 blue. Your black doe X blue buck will on average give %50 blue.
On the next generation if you use the black buck then some of the offspring might not have a carrier so some children will not be able to produce blue babies. Of you use the blue buck then even if the off spring are black they should carrier a dilute gene so they could have blue offspring if they are mated with a dilute caring rabbit.