I do this with some sort of drano max gel pipe clog remover. It's bleach for sure. I tape the negative down with quality masking tape and take it into the darkroom.

In the sink, I use a pipette to put maybe 5-6 ML of drano bleach onto it. I spread it around with a paper towel. It's ready to come off in about a minute. I wipe it off, rinse it real clean. After I'm sure the bleach is rinsed away, I put it in a tray of cold cold water, and rub the dried snot off the emulsion side. It comes off best in cold water.

The negative is good to scan. It's also pretty good for alt process. It does seem to block UV light somewhat, but an extra stop or two exposure and it will still produce fine results. The brown print below was just a test on a paper which I determined wasn't a good choice for this work. The midtones and shadows printed real nice for a cyanotype on the first try.