It would be easier to take care of her. By living on the Dawn-Lynn Dairy, Jen had learned many things about cows. She learned that it takes less work to keep black cows looking nice. A cow's black hair is shorter. It is easier to clean.
White hair is longer.
It has to be scrubbed more, and clipped. A white cow out in a field looks dirty very quickly. Blackie stood quietly in her stall. She did not pay any attention to the people who were looking at her. She did not even seem to see Jen.