Dog has discoloration around genital area, getting concerned?

Please read whole question before answering, thanks.

He has a vet appointment Thursday, but I am very confused as I have never seen or heard of anything like this. My Shiba Inu's lower abdominal area and genital area, including penis is always a light pink color...it is now a dark gray color, it looks as if he rolled around in ashes or something (which he has not because he doesn't get into anything). He's strictly an indoor dog and doesn't get muddy (so please don't tell me "he's just dirty"). He has been licking around his anal area as well, which I know could be an anal gland issue, but in relation to the skin discoloration, could it be related?

Also, if it helps, he was sick about 2 weeks ago because he had gotten into some chocolate and had bad diarrhea for about 5 days, took him to the vet, got Flagyl which cleared everything up and he's now back to acting himself.

It's actually disturbing looking, almost like a huge bruise, but he is acting 100% normal. The area doesn't seem tender/sensitive/painful at all as he allowed me to check it out and stuff. He's alert, playing, eating, drinking, solid stools, peeing normal - no sign of pain or illness at all (besides the occasion licking of the behind).

So all in all his light pink skin in his abdominal/genital area has turned an ashy grey and definitely doesn't look normal. WHAT COULD IT BE!? Thank you.