I’ve used warm soapy water to remove the blush, although WS says soap is not required or desired. Warm water scrubbed with a scotchbrite pad should do what you need. Peel ply (Dacron cloth) will minimize the amount of blush, but not completely eliminate it, so some warm water scrubbing is a good idea if you plan on laying another layer, or applying some sort of finish coat. You can sand it off, but it loads up the sand paper pretty quickly, and you might not get it off completely.
Once the blush is removed, how do you get the nice shiny finish on the carbon fiber?
Can I use wet sand up through 2000 grit then cut & buff?
Or use a better, more modern epoxy resin that does not blush. I have been using System 3 Silvertip epoxy for many years and it does not blush.
I saw some photos of your plenum earlier on and it looked very nice and I didn't notice any blush or blemishes, so has it developed since then?