Yehuda (who is currently in Canada) included this picture on a recent blog post... this is precisely what I've been looking for all around the world, although alas not yet successfully. It's a grey squirrel, but with black fur. There are populations in several places (including Canada), but I have not been lucky enough to find one yet. I must find an excuse to visit Ontario again... If anyone else has black squirrels in their part of the world, please let me know!
UPDATE: I have started a new blog for black squirrel sightings and other sciurophile nonsense. Please go to Shadowtail to report any black squirrels in your area! Thank you!