I have a water blister on my thigh.4 days, then turned sort of flat and darkish, and feels hard and rough. it's been 12 days now. thoughts?

YerAPlasticSurgeon!!: It's most probably healing. The fluid inside either leaked out or was resorbed and now the small layer of skin is adherent to the underlying base of the blister, forming a thin scab which will eventually slough off as the underlying tissue is rejuvenated. This is the basic process that happens with cosmetic peels (for wrinkles and dark spots).The area will be sensitive to sunlight so use sun block

Hard to say: Most blisters flatten and get hard and rough and some darken before the skin falls off, but 12 days is a long time. If it is bothering you at all, you should either get a consult and send a photo to one of us, or see a doc. Meanwhile, no harm in warm compressing. If you put a slightly damp gauze over it and change every 12 hours, pulling off any dead dry skin, it might resolve sooner. Hope so!