Add strobbing(blank frames) to videos

[deama]

Hi so I'm trying to add a strobbing effect to videos, so strobbing is basically inserting a blank(black) frame after each real frame, e.g. Real frame > Black frame > Real Frame > Black frame etc...
As far as I can tell, I think I have to use either avisynth or vapoursynth for this? There's no native option for this?
Anyway, I made this script for avisynth:

blackframe=clp.blankclip()
interleave(clp, blackframe)

I insert the above in the custom section, with the option to insert at End.
Ok so the problem is that, it works, but it's about half the speed as it should be. I googled around and found a function to speedup the video, with AssumeFPS? So I tried this:

AssumeFPS(60)
blackframe=clp.blankclip()
interleave(clp, blackframe)

but this doesn't work. Anyone got any ideas?

I also tried vapoursynth, but for some reason when I try to insert a script in custom section, it always crashes, no matter the script, even if I disable the custom section it still crashes, I have to restart hybrid (or switch to avisynth then back to vapoursynth) for hybrid to work, but vapoursynth still won't work. Here's the vapoursynth script for reference:

import vapoursynth as vs

core = vs.get_core()
clip = core.std.AssumeFPS(video_in, fpsnum=24)
blank = core.std.BlankClip(clip)
clip = core.std.Interleave(clips = [blank, clip, blank, clip, blank, clip])

clip.set_output()