Like if a character is hit by a blindness spell or something, and can't "look at" things and it'll just say "you can't see it"

You could modify the lookat command so that it checks for that.

Or you could create your own look at command with the same pattern; because custom commands override the ones declared in libraries. You could make a custom "look at" command which just says "You can't see."; put that command in a container somewhere the player can't reach it, so it won't do anything. Then when the player is blinded, you can enable your new command by using the RemoveObjectfunction on it; and when they can see again, you could remove it usingAddToInventory`.

Alternatively you could use the 'darkness' mechanic. Make every room dark, and the player a light source which switches off when they are blinded. Changing some of the messages might do something like what you want.