I use a lot of feedback in my patches and flashing occurs a lot of the time. This would be a critical issue at an installation as it can cause people to have seizures.
One solution is to have a black video channel ready to mix with or fade to, but I tend to want all the channels I have available for other things.
Is there any other way to reduce this? Any type of filters for toning everything down quickly if needed?
Thanks!
Flashing or strobing can come from many different sources. If you post a video of your worst culprit and how you’re creating it, it will allow us to suggest ways to tone it down. Video paths that invert the video will tend to have more strobing in their feedback, so maybe turn any inversion off and see if that helps.
There’s no one stop “seizure prevention” filter you can run on video that I know of. Best advice I can think of would be to put a seizure disclaimer/warning at your installation before people enter.
The seizure warning is probably the best idea.
If I find a particular set up introduces a lot of flashing will make a thread about it thanks very much 
Photosensitive Epilepsy is not that common (affecting 3% of epilepsy suffers). That said it’s always a good practice to warn folks of strobing lights.
What is more common is annoying people or giving sensitive folks headaches 
There are a few kinds of feedback, without knowing your workflow here are a couple of tips:
Attenuate the signal - either with your mixer (this is what you’re doing mixing in black) or once the signal is inside your rig with attenuveters on the modules.
Don’t use feedback as the direct output. I like to use feedback signals as modulators, so I’ll have a feedback loop patched into the system but the feedback does NOT go to main output, it’s used to modulate or affect other signals in the patch.
Adjust your monitor - It’ a critical component in the feedback path and by adjusting brightness, contrast, color and ambient light you can greatly affect the intensity of feedback
My most recently uploaded video uses heavy video feedback (two different fb sessions, blended with soft-keying using a third, generated video stream) in FFMPEG, so I used the photosensitivity filter to tame the oscillations so the foreground stands out better. I should post the video itself in the gallery section.
Anyway, this filter works very well. I upscaled to HD before applying it, to exploit the higher resolution, but this increased the processing cost enormously. I’ve compared the SD and HD versions, and I’m uncertain the extra processing cost was justified, but as I like upscaling to HD for publishing, it’s not a big deal for me. Increasing the frame rate might be even better, but I expect that’ll double the processing cost. That’ll make it an overnight job! Not a problem for me, as I’m scripting all the post-processing. So I can run different versions as many times as I like. That’s a big part of why I use FFMPEG. I’m a programmer, so that’s easy for me. YMMV but a decent video editor should have a similar filter and may even offer scripting. I know there’s at least one editor that has that feature, but its expensive.
My point is that you can do this in post-processing. Whether you do it in post, and how you do it, is up to you. I agree with all the advise given here already - I’m just adding my recent experience with some free software.
BTW, I applied the photosensitivity filter to the inputs to the filtergraph, not the output, as that produced a very different result. I.e. I lost all the animation. That’s how powerful that filter can be. I expect similar filters in other software will work the same way.
My general experience is this: where you place some processing in the whole system will be important, whether its hardware or software, production or post-production, whether its audio, video or whatever.