Are you sure it's not just where the elbow has a hard patch when they lie down on concrete or other hard/rough surfaces? Is there one on both elbows? Does your dog experience pain when you touch or move the skin in that area?
It does seem a little more red than I'm used to seeing in other elbow patches, so it might need to be seen by a vet if it's not just a callus.
It seems to be painless and not causing him any discomfort. However it appears to be getting larger with a slight black crust around the edge of the growth.
Yes definitely, a vet's opinion will be obtained.
Yes, the different colors on the inside and outer edges may very well be a warning sign. Out of curiosity, have you been watching it over a period of time? Please let us know what the vet finds out. Your story could help someone else down the road!