I have a Cardigan Corgi. I noticed her fur changes colour from white to black and I guess its normal? But I want to ask. Does your dog's whiskers change from white to black colour? Mine does and I wonder why. Then I guess it will fall off after awhile and grow again? I'm not too sure about that.
I'd suggest to wait and observe your dog. Perhaps this is normal for your dog, but if it is causing problems for your dog, I suggest you go to the vet. If it is causing no problems than perhaps it is not something to worry about. Keep watching your dog and you'll find out.