To get a blue pitbull from breeding your white female pitbull, you would need to breed her with a male pitbull that carries the recessive gene for the blue coat color. Blue pitbulls are a result of a combination of the dominant black gene and the recessive blue gene. Black pitbulls have two copies of the black gene (B/B) while blue pitbulls have one copy of the black gene and one copy of the blue gene (B/bl).
Therefore, to increase the chances of getting a blue pitbull puppy from your white female, you would need to breed her with a blue pitbull male (B/bl) or a black pitbull male that is known to carry the recessive blue gene (B/?bl). By doing so, you have a higher probability of producing blue pitbull puppies, although the actual colors and coat patterns of the puppies can still vary.