While not often actually silk, they tend to be soft polyester and use fabric filler rather than near-credit-card-grade plastic. (I agree, I don't understand firm sleep-masks)
In the photos (I know they're photoshopped these days), look for the mask to actually wrap around the user's face, and look for photos that show the same material on both sides (so you can see that you're not getting a different material on the user's face-side). Look for a bit of gloss to the material so that it appears to be a nice silk-or-polyester. Look for the little plastic clip of an adjustable elastic loop so you can adjust the tension.
I definitely have had to play with the elastic tension to get a soft fit.
Edit: better example: https://smile.amazon.com/Adjustable-Lightweight-Comfortable-...
An almost-meets-the-cut-example-but-might-have-matte-fabric: https://smile.amazon.com/ZIMASILK-Natural-Adjustable-Super-S...
A generic search: https://smile.amazon.com/s?k=silk+sleep+mask&crid=1X69JJEXPF...
https://www.amazon.com/dp/B07PXB4NZB/
(no affiliation, just a satisfied user of the product)
(I previously used a sleep mask bought on Amazon but found it slipped off too often. Also, the straps irritated my ears.)
I've had three in that time due to accidentally leaving it in hotel rooms.
A test of a good mask is if you can see any difference with your eyes open or closed, and if you can feel your eyelashes touch anything if you open and close them. These are excellent and have a satin outer which slides around the pillow rather than coming off or moving.
They include ear pads to block out sound, but I find them a bit uncomfortable and never use them.