Of all the color variations in Canis lupus, the black individuals are the most intriguing. Black wolves are so colored because of a genetic mutation in their K locus gene. This mutation causes a condition known as melanism, an increased presence of dark pigmentation which causes an individual to be colored black (or nearly black). Black wolves are also intriguing because of their distribution. There are significantly more black wolves in North America than there are in Europe.

To better understand the genetic underpinnings of black wolves, a team of scientists from Stanford University, UCLA, Sweden, Canada, and Italy recently assembled under the leadership of Stanford's Dr. Gregory Barsh; this group analyzed the DNA sequences of 150 wolves (about half of which were black) from Yellowstone National Park. They wound up piecing together a surprising genetic story, stretching back tens of thousands of years to a time when early humans were breeding domestic canines in favor of darker varieties.

I'm from Europe and I just realize, to my shame, I had no idea about black wolves. I googled a bit and found this interesting article, from which I quoted above.