You will need to install a user script manager extension to install this script.
Basically, Netflix doesn't give you any way to mark stuff you've seen making it really difficult to keep track and not accidentally watch stuff multiple times. It's obnoxious to say the least. Until they get their act together, here's a very basic GM script to add a "watched this" button that you can click to toggle shows to go semi-transparent.
Granted, it would be better if I could remove the and have them replaced with something you haven't seen, but that would require Netflix code or API work. This is at least better than nothing.