Never been a recorded melanistic mountain lion in North America, period. Highly doubtful the sighting in the original post is legitimate. When single animals wander into Minnesota/Wisconsin, where there are also no breeding populations, there are dozens of sightings, pictures, etc to corroborate with 2 to 3 times less population density. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.