[masraum]

boiled linseed darkens over time, doesn't it?

Tung Oil (the real stuff which is hard to find) hardens like linseed oil, and is not supposed to darken. I believe it's supposed to waterproof wood and create a natural appearing (more matte than gloss) finish.
THis is a source for the real stuff. I'm not recommending this as I don't have enough experience. I'm just offering up the info.
https://www.realmilkpaint.com/shop/oils/pure-tung-chinawood/