Well, colour me baffled. I've got a nice background graphic. On a new layer, I add some greenscreen footage of a guitarist. I set the blend mode to Alpha Stencil so that I can use his (moving) silhouette to contain some movie footage. Then I duplicate the guitarist layer, but this time change the blend mode to normal, and lower the opacity, so that the silhouette contains a mix of him and the movie footage.
On Tuesday, I do this and it looks just fine. I do the same thing again today, and for some reason everything around the silhouetted guitar player goes black. I want it to be transparent, as it was on Tuesday!
My kingdom for an explanation! (I have a very small kingdom.)