.. Hello Alan!
I´m not sure if there is a way to do that, but here are some ideas for further versions:
By working on a project i used 3 Layers:
1- Original video footage with ONE blocker (some houses)
2- Just a Blocker for some flames
3- Another Blocker for more flames :)
When i use a blocker on the Layers 2/3 the blocker become BLACK- because there is no background loaded.
When i take my footage as background, the blocker from layer one is not visable.
The idea is to "lock" the layers to one layer-background. So the blockers of the other layers are not black... do you understand what i mean.
The 2nd thing is this:
I use my blockers with an offset-txt (some tracking data).
So i make the flames burn behind some houses.
But there is this HARD EDGE. A feature "blur blocker" would be fine, so you have a smooth edge ... and you also don´t need perfect tracking-information, because the blur would give a little tolerance in movement.