Hmmm... I was going to suggest seeing a Urologist to help determine what this is, but it sounds like you already did & he didn’t know. This may be just a congenital discoloration that’s probably harmless. It sounds like it’s not causing you any pain or discomfort, but more a case of being unhappy with the way it looks. I can certainly understand that - if it were me, I think I’d feel the same way. I suggest maybe seeing a different Urologist & ask if there’s anything to be co concerned about & I imagine you’d also want to ask if there’s any type of cosmetic surgery available that could correct the discoloration.