The visual deflicker works very well on most shots but I often shoot flowers blooming on a black background and the deflicker really doesn't work on these shots. I think the issue might be that there isn't much flicker on the bg because it's so black. I've tried using the drag box to isolate smaller areas that contain more information but nothing really seems to work. I've attached a ref image.

I dark regions usually there is a lot of "technical" flicker that you can't see. for such images, I'd recommend to set the reference area to a place on the flower. Try not to cover the black background. If necessary, you can animate the reference area, if the flower moves. The reference area can be "keyframed" - just draw another reference area on another image preview. You'll be then asked if you want to animate the reference.

ok thank you. I'll try that.

I figured out a work around for this. It's a simple cheat. The issue was my images contain either black or a moving flower. Deflicker did not work on either of those things. So I have to add a static neutral coloured object to the shot that would be removed later. This could even just be a tripod leg on the edge of frame. I then use this area as reference area for the deflicker and then crop it out after. It works perfectly.

That's a good approach for such scenes.