Sorry, I forgot to mention this is AE CS4...
Put the blob on top of the film strip, set the filmstrip's matte mode to Alpha or Luma Matte (hit F4, if they are not visible initially).
Thank you very much! Exactly what I needed.Now I just need to make the edges feathered...
One of the best sayings I have heard about this kind of stuff is, "you don't know what it is that you don't know"!!!
Appreciate the help!
If you just want a general feather of your mask, apply a blur to the matte layer (the blob). If you want something that that preserves the detail of your blob's edges, precompose the blob layer, then in the precomp duplicate the layer, blur the top one, and set it to an appropriate Transfer mode to blend with the lower layer. Adjust blur and opacity of the top layer to taste.