Why do some cows have black blotches?

Answer

It is entirely based on the cow's genetics and breed. They have spots because their parents had spots or maybe the cow's parents were different breeds and the resulting mixture was spotted colouration. The breed you are thinking of that has spotted cows would most likely be Holstein. Other breeds include Milking Shorthorn, Shorthorn, Texas Longhorn, English Longhorn, Normande, Florida Cracker, Speckle Park, Beefmaster, Simmental-Fleckvieh, Ayrshire, Guernsey, and many more.

If you are talking about a different type of spot like warts or ringworm or mange then your cow is in need of a veterinarian.