Well... Although I've never done this with a model, I've done a few things like this.
One of the techniques I use often is to put an exact duplicate of the footage above, cut a mask around the blemish, and then modify the whole layer below until the blemish disappears.
So, it would look like this:
Layer 1: -----------------| |----------------
Layer 2: ----------------------------------------
If, say, the blemish were darker than the skin, I'd adjust the levels of layer 2 until the part of it visible through the mask on layer 1 looked where I wanted it.
Once you have that, then you keyframe your mask shape every few frames.
I've changed the colors of entire flying severed heads this way.
Be sure your mask has fuzzy edges, though.
![[monkey] [monkey] [monkey]](/data/assets/smilies/monkey.gif)
Edward
"Cut a hole in the door. Hang a flap. Criminy, why didn't I think of this earlier?!" -- inventor of the cat door