Have blue pitbulls ever been born with a brown coat?

No, it's not possible for a blue pitbull to be born with a brown coat. Blue pitbulls always have a blue or gray coat due to the genetics responsible for the blue color dilution. They carry the recessive "d" allele for dilution, which causes the original black coat color to be diluted into a blue hue. The brown coat color is determined by a different set of genes that do not involve the dilution gene, so a blue pitbull cannot have a brown coat.