I could be a case in point. Now granted, I do not have an "official" diagnosis. (my husband was laid off at the time.) My rash was my "turning nifty fifty" present. It started on one ankle, then the other. Not my elbows or knees!
Why do I think it was Dh? It was extremely itchy, so much so that I would wake up in the middle of the night, unconsciously scratching. And the spots were very red. Not all of them were always blistery - another thing that didn't fit perfectly.
So why do I think this was DH? I had the rash for 10 months and NOTHING helped it, it only got worse. It was when my daughter went on a gluten free diet - for completely different issues - that the light bulb went off in my head. After reading and researching, I went gluten free. Gradually the rash began to heal. More telling, a month into the diet, I "went off the wagon" and within 2 days had a fresh crop of itcy witchy blisters.
As my rash healed, I was left with reddish purply spots that took months to fade. Two years later, most of them are gone now.
Self diagnosing is risky, but i didn't have the dollars at the time to throw at it. From what I have read, the way my rash began may be less than a classic presentation. But, the time course and manner of recovery after going gluten free is more typical.
So yes, I think there is the potential for DH being more common, and less recognized, than some numbers indicate.