Kirsten, I wouldn't worry about it either. When I first adopted Sunny he had black stuff on his nostrils and also waxy ears. My vet said that this could be allergy related. It could also be a result of food allergies. Now he's fine so I think it could have been related to his previous diet.
Now my newest furkid Sky has the same thing. His ears are much better but he still gets the black stuff in his nose. I'm sure it'll go away in time.