Campbells dwarfs have a gene called the diluting gene.
This gene "dilutes" or lightens the colors of the coat dramatically. However, its only been physically seen in Platinum and arguably Blacks. Right now its still very new and requires much more research, most believe that a lightening/browning Black is just due to aging and not the diluting gene. I believe this too. Ive done extensive research in this gene and it is not shown in Blacks. It's just bad breeding.
However extreme cases where pitch black babies turn into lightly colored fawns at adult hood make everyone question. I have a male now that makes me look back at my research for holes!
I think however it's a matter of the individuals.
Hes almost blond now!
Blacks also have a "silvering" gene which adds white into the coat during aging. All Blacks will have this. It depends on genetics if it appears harshly or seemingly not at all. This very normal.
So yes, this changing of colors is normal.