I had blonde hair when I was a kid, black hair after about 7 years old, and turned gray around 55. I don't know how old your cow is, but age may cause hair to change colors. also my wife is blonde in the summer when she is out in the sun but it turns darker in the winter to a more dirty blonde color, so weather may factor in. I would be interested to see if it stays black or turns back to white again. different stages of pregnancy can change color also. If she seems healthy, I wouldn't worry about it. I would be interested to hear what color her calf is; especially if she is bred to a red bull.