Hold on, what editing system are you on?
Because if you are on Adobe Premiere or FCP, there is a much better way to do film burns than just turning down the opacity of the film burn clip.
Lay the film burn clip over top of the clips you want it on, and then look for an effect called a LUMA KEY. Apply this effect to the film burn clip, and then on Adobe (not too sure on FCP), you turn the threshold (the top slider under effect controls) to 0% and then the cutoff slider, directly under the threshold slider, to 99% and this will create the desired look.
The reason I say this is the better way to do the effect is because although reducing the opacity of the film burn will sort of work, it will make the final clip look different, and really effect the white levels of that clip, and the you have to mess around with colour correction in order to return the clip to its original brightness. Just go for the luma key effect, its a much better way of doing it.
