I think the scientific article keeps this moderately in perspective. The only difference between these and the grey is that they have more melanin (black pigment) in their fur. There is no excess testosterone - that is reporting hype. There may be some sexual selection that the females prefer the black male. But that is it. Not more aggressive, randy or any such thing. Not out competing. Anyway, thats my take on it.
The worldwide reports are interesting. The Canadian ones would be a black mutation of the USA grey. The Eastern European ones would most likely be black versions of the red or European squirrel.