If you feed out of a plastic dish he may be reacting to the chemicals in the bowl. Swapping to stainless or ceramic bowls may solve the problem.
Also if he is in a crate or pen and puts his nose under trying to get out that might wear the black off.
And some dogs develop a lighter nose in winter.
Its called snow nose.
You can read about noses here
www.canismajor.com/dog/nose.html
If it looks like it might be infected you can try some triple antibiotic or neosporin ointment on it.
If it won't clear up see your vet just in case it is something more serious.
Hope this helps you!
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