Boxer fur loss

Question
I have a 2 yr old fawn male boxer.  Over the past couple months his fur started to thin on his flanks (right > left).  These areas now seem almost bald and has an underlying blackened skin underneath.  No itching seems to be involved, no diet changes, no change in energy level, etc.  The vet scrapped his side and tested for mange and tested his thyroid function, both negative.  
There didn't seem to be any concern for hyperadrenocorticism...he's not having any other symptoms of this disease i.e. increase thirst,urination,hunger,etc.
I also read that it could just be 'seasonal flank alopecia' Is this something of concern?  Should we still seek a vet dermatologist?

Answer
Yes, I would seek out a veterinary dermatologist as something is obviously wrong with the dog. http://www.acvd.org/

Did your vet just run a basic thyroid test, or was a six-panel (T4, T3, Free T4, Free T3, T4 Antibodies and T3 Antibodies) test run? The more involved test usually has to be sent out to an outside laboratory and it is much more accurate than the basic test.

Yes, this could possibly be seasonal flank alopecia, but that would have to be determined by a veterinary dermatologist. http://www.hilltopanimalhospital.com/seasonalalopecia.htm