Old wounds + hair growth

Reading jessica's post about Rileys hair growing back made me think about one of teagans wounds...

Last year, where she had a pressure sore...the hair has never grown back.
All the rest of the hair has grown back from her operation, but this one patch hasn't grown back.

Do you think that the pressure sore may have just been so deep it totally ruined the hair follicles?

This was the sore when she came back from having it treated for 3 weeks in hospital (last year).
Now the skin is dark coloured (where she should have dark hair), but doesn't have a single hair on it.

I know we used to use what was called "green oils" on bare patches (such as saddle patches) on a horse, and the hair always grew back in the bald patches? There is another name for it, but I can't remember the correct name as the one from our vet was called this. It might come to me, but don't hold your breath! It began with derma......dermoblin, or something like that, sorry I will try and remember it!