Cat with blisters on hind legs

Our female siamese (Sealpoint) cat has had blisters appear all over her hind legs.  We've had her to the vet but they can't seem to find the cause.  We've treated her numorous times for fleas using revalution and don't see any fleas at this point.  She's an inside cat and our only pet.  She constantly licks at the places but doesn't seem to be in any real pain from them. Her weight, appitite is good and she doesn't act sick.  The blisters/sores start at her ankels and go up to her inner thighs.  Her hair there had fallen out but is growing back somewhat.  Any suggestions??  We're just at a loss.  Thank you.

[Aleda M Cheng, D.V.M., C.V.A]

Dear Rod,

Your cat could still have flea allergy dermatitis among other things.  A single fleabite, even if you never see the flea could cause symptoms for a month.  Your cat