Some sort of rash - any idea what it is?

I have a 7 year old Lhasa Apso - he has developed some sort of rash (I think it is?) on his inner thigh. It's pretty large, taking up about half his leg. It appears to be black or very dark gray in color and he keeps licking at it so it must be irritating him. Do you have any idea what this could be or what it could be caused by? I'm taking him to the vet this week but I just wanted an insight on what the possible cause for this weird looking rash could be.

Really too hard to tell w/o seeing it and knowing more of the dogs environment.

Can be allergy to food/environment/allergies to grass/pollen/chemicals used in the home to clean floors/ flea allergy ..just about anything. If it's irritating poochie and you don't know what it is make that appt to see the vet.

You could also post a pic of it here incase some of us can see if we've seen that 'type' of rash/allergy/bite before.