Surely not, I thought.
But surely yes, says Wikipedia.
And horror of horrors, says the Daily Mail:
Scientists say the testosterone-charged black is fitter, faster and more fiercely competitive than both reds or greys. … It has already taken over in parts of England and appears to be spreading. … "They could overrun most of the Eastern counties within ten years."
Just so that you can calibrate the exact type of gibbering paranoia appropriate to this news, I should clarify that the black squirrels are mutants rather than immigrants. Although the grey squirrels that they’re mutated from were themselves immigrants.