they are not cross breeds they are either a grey squirrel or fox squirrel in the black phase. That is a fox squirrel in black phase looking at the tail.
x2
To be exact that is a eastern fox squirrel Sciurus
niger. The latin name was given for a reason. The black phase is one of many phases that vary from one geographic location to the next. The black phases seem to be the dominant DNA trait in the fox squirrels and working its way northward from the south.
In some markets ie Tulsa the fox squirrel is found more often in attics then the eastern gray squirrel any other part of the country. Locally the gray squirrel is never found in the same location as the fox squirrel. The gray squirrels are very territorial and won't let them in to co exist so I doubt they are even thinking about doing the naughty and making a new mixed squirrel races.
My best guess is it not crossbreeding at all; we are watching a species evolve to survive from an increase in predator populations.