I understand how you can be concerned about this.
I believe its some sort of skin discoloration and i don't believe that this is anything to worry about.
From what i have read it seems as though you have some sort of melanism its the opposite of vitiligo where they are white patches of skin rather than a dark patch or patches in your case.
If your GP said that its nothing to worry about and you feel you NEED to go get a second opinion then go to a urologist and have it looked at.
Best of luck to you hun!