Using idioms such as "the pot calling the kettle black" in fantasy novels considered to be something that breaks immersion? In a fantasy setting, you wouldn't expect that particular expression to be used, especially if the people in that world don't speak English, and are not related to European or even don't look like Europeans at all. Is it considered a bad thing? If so, how much? As I recall, I don't think expressions like "the pot calling the kettle black" was never used in Lord of The Ring, so I would expect that it's something you should avoid. Is there a rule or a set of rules, hardcore novelists like Tolkien adopt when writing a fantasy or even a sci-fi book in order to not break immersion? I am thinking some proverbs and idioms need to be avoided in order to be 100% lore-friendly, realistic, etc.
Actually, the pot calling the kettle black is exactly the idiom you would expect. Both are soot-stained from the fire, after all.
The idioms to beware of are those that don't make sense in your world. Ideally, all of them, but most readers will let your archer fire arrows in an era without gunpowder, or have a strong suit in a world without cheap paper, and therefore without playing cards, and therefore without the game of bridge.
But you can't have someone have a cog loose in a world without clockwork or gearing. Or maintain radio silence by not sending letters.
And you should also beware of idioms with apparently modern meanings. A character can cast broadly over the field when sowing beans, but if he then broadcasts a rumor, people are going to be jolted.