Replace all completely black frames with interpolated frames
After poring over the documentation all weekend, I figured out how to use the minterpolate filter to do motion interpolation.  The full command is

ffmpeg -i in.mp4 -vf blackframe=0,metadata=select:key=lavfi.blackframe.pblack:value=50:function=less,minterpolate=mi_mode=mci:mc_mode=aobmc:me_mode=bidir:vsbmc=1:me=fss -c:a copy out.mp4


These were the settings that gave me the best-looking results.  If these are too slow for you, you can try something like

ffmpeg -i in.mp4 -vf blackframe=0,metadata=select:key=lavfi.blackframe.pblack:value=50:function=less,minterpolate=mi_mode=mci:mc_mode=obmc:me_mode=bilat:vsbmc=0:me=tdls -c:a copy out.mp4


which went about 3x as fast in my testing.  You can find the documentation on the different minterpolate options here.