Been burned by this a few times. Workflow runs green all the way, progress bar fills up, but the preview is just pure black. No errors in the terminal either, just one quiet warning line. Turns out that line is the whole answer—KSampler calculated NaN in the latent, and when it converts to an 8-bit image, every pixel collapses to zero, so it’s black.
Sometimes it’s gray or has a green tint, same deal, usually comes down to precision. Don’t touch any settings first—go check that warning line in the terminal to confirm if it’s NaN. If not, it’s a different issue. Once confirmed, handle it based on the precision flag for your card and model.
Sometimes a single launch parameter fixes it, other times you need to download a separate 200+ MB VAE. My own 16-series card loves pulling this crap. Start with the fastest troubleshooting steps, don’t jump straight to redownloading the model.