Hi XXXXXXXXX
As long as your cat is showing no signs of ill health or discomfort and as long as the black areas do not appear to be swollen or rough on the surface this is probably nothing to be concerned about.
Pigment changes occur frequently in dogs and cats both on the skin and on mucus membranes (lining of the mouth, gums and tongue). It is not peculiar to the Sphynx breed. The causes are not clear. It can involve loss of pigment as well as development of pigment and occurs more frequently as an animal is maturing.
So if everything seems okay with him don’t get worried. If you feel there is more to it than that, please don’t hesitate to contact me again
Regards, XXXXX
Cat Veterinarian
Bachelor of Veterinary Science (University of Melbourne, Australia)