Lump on tail that's growing

I adopted Charlie back in 2013 (as an adult of unknown age) - and he's been as healthy as can be until the past few weeks. Slowly, he started developing a lump on his tail... and today, I found a dark spot on his lump.

Until now, the lump hasn't seemingly bothered him - it wasn't sensitive to the touch, and he eats and drinks like a champ. I am super scared about this black spot though.

Any thoughts or ideas are appreciated!