Thanks Cory.
Codec: QT PhotoJPEG, 25 fps, 1920×1080, exported from Final Cut (on a mac), and then imported to a PC where After Effects is used.
In an NLE: In Avid (PC), the clip in question does NOT show the black frames, so this is something to do with how AE (PC) is interpreting this footage. I’ve played with the interp settings to no avail.
Reason I’m not using original, pre-trimmed clip in AE: Because the original clip DOES have “real” blacks in them, which I don’t want, and trimming them in AE on the timeline means that when I play with Timewarp and reverse the clip, etc., those blacks come back to haunt me, even if I trim them out on the timeline to begin with. Maybe if I precomp stuff this won’t happen.