As the article suggests, these seem to be just a natural variation of the ordinary grey squirrel. Nothing new there, other than the numbers that seem to now be in evidence, perhaps due to the females prefering the black males as partners resulting in higher numbers of black squirrel births.
On a large Estate I worked on in Bedfordshire, where I did pest control in my free time, we had black 'greys' nearly 20 years ago. I suspect the population of them there may have gone the same way as we are seeing now, had it not been for the fact that all squirrels (no reds present) were regarded as pests and the black ones did tend to 'stand out from the crowd' so to speak, and found themselves on the wrong end of a rifle barrel as easily but perhaps more often than their plain grey cousins. BTW, they taste excatly he same as a normal grey