"Blue nose Pitbull" is a word for a blue/greyish dog. Both the blue gene and the rednose gene are recessive, created by two different genes, that washes out the coat color. The blue gene turns black to gray, not just the hair, but the skin also, so a blue dog will have a slate gray nose, lips and eyelids instead of the black color. 

The red gene washes out black to a brown or chocolate color, not like the blue gene, which affects black colors, the brown gene also turns the brown to an orange/cinnamon color, on skin, hair and nose. The more generations breed like this, more pigment is taken out, which would eventually create skin problems.

Below is a picture of a Blue Nose Pitbull.