Is it really worth adding a whole material property/game rule just to let creatures evade ESP when you could add an item that does the same thing in a way that's more consistent with the existing game rules?
Does the BlockESP flag prevent creatures from using ESP, or just being detected? Why?
About 1 in every 41 plain helmets are tin (no other headgear is generated as tin), and by the time the player has high enough INT to use ESP reliably almost every humanoid NPC will reject tin helmets because there are better options strewn around the dungeon floor. Will players ever fail to detect a threatening NPC because they found a tin helmet?
Does this change add more depth or more complexity?

