My dog looks just like a Border Collie, but you didn’t find that breed.

Black coats with white markings can come from a large number of different breeds. And because the black fur and white markings are inherited separately from each other, your dog’s parents may have been a different color entirely. For example, a mix of Labrador Retriever and American Pit Bull Terrier could look quite a bit like a Border Collie; the black coat could come from either breed, and Pits frequently have white markings. 

Though it might be surprising to hear, a lot of people (even dog experts) mistake black and white dogs for Border Collie mixes. Importantly, however, black coats with white markings are by no means exclusive to this breed. The genotypes necessary for this phenotype can be inherited from any number of breeds.