Blackmagic Forum

This issue occurs with ultra keyer and the delta keyer, and presumably any other function that uses the eye dropper...

I've had a similar issue, holding down the alt key sometimes seems to help.

If you're previewing a node downstream and picking color off it, the keyer is caught in a cycle - it picks color from a preview that it affects ... and that affects the color it picks next. Hence the flicker. This behavior is in line with how Fusion 9 works.

You could switch the preview to sample the node feeding into the keyer, or use Alt-drag to force the color picker to act on the upstream image.

From the Fusion manual:

Use the Pick Button on the Background Color to select the the blue or green screen color from the image. Hold Option (Alt) while click dragging the pick and it will pick the color from the upstream image, making the key not flicker.