"Brindle" is a particular kind of striping pattern on a dog's coat. "Blue" refers to a rather rare dog coat color. A blue brindle pit bull is one that displays striped fur with at least one of the stripe colors being blue.
Blue coats aren't actually blue. They are a soft grey similar to the color of stainless steel, which appears blue in certain lighting conditions due to the sheen of the coat. This is a common mutation amongst certain breeds, but sparse in comparison to the entire species. In a dog exhibiting brindle characteristics, the primary color determines how blue the striping appears.