black crusty rash on french bulldog

Question
Thanks for your time!  My four year old frenchie (neutered male) has been battling a rash for over 6 months now.  My vet is completely stumped, and I am at my wits end.  I will try and describe it as best as possible:

When it first started, it was only on his belly.  He will get a blackish crusty spot.  When I clean and remove the crust, it is very red underneath, and the hair falls out.  He doesn't like me touching the spots, but never yelps, I think he just hates sitting still.  These are circular spots, from dime sized to Quarter size.  Later in the summer, he started getting them on his back.  It looks like he has a lump under his fur, when you part the fur, the same crust is there, again, I remove the crust, and then the hair is gone.  At one point he had eight of these spots at once.

Right now he is on Cephalexin for a greenish penile discharge.  Our vet thought maybe it would help with the rash, too.  It seemed like it was helping, but then this morning I found another black crusty spot on his belly.

Also, our vet sent out a sample to be tested, to see if it was fungal in nature.  It is not. He is on a raw food diet, and is otherwise in excellent health, very energetic, and vibrant. He has been on the raw food diet for about a year.  I use Maleseb shampoo, is this okay?

Any help you can give would be much appreciated!

Sincerely,
Denise Lyding and "Rufus"

Answer
I think that what you are describing is a pyoderma.  That means that the skin is infected with something causing pus to form.  That is all of the crust you are seeing.  Your vet is on the right track with the cephalexin and it may take 4-6 weeks of therapy to completely get rid of it. (The green penile discharge is normal in many dogs.) Sometimes there is also a fungal skin infection that the Malaseb may help.  If the dog is itchy, it could be from allergies, which are not being treated according to what you have written. Ask your vet if your dog might be allergic.