I remember reading about the upas tree back in my school days, the locals perpetuated the story that the tree was so poisonous that no other living thing, plant or otherwise could live within a few meters radius of the tree
I have seen blades poisoned in the Philippines but I am not convinced by the method used. They fry poisonous spiders and worms together and then add acid. The blade is then dipped in this mixture and is considered poisoned. The thing is many venoms are protein based. If you expose the venom to high temperatures surly it will breakdown the protein and render the venom harmless. I think the same with plant toxins. If they are used during the forging of the blade when it is red hot then surly it will destroy the toxin.
If as they say the toxin was added during the forging than perhaps its as simple as quenching the blade in a strong solution of arsenic.
I don’t think it is a common practice and perhaps it was only done during battles but I don’t think it impossible since this has been done in several cultures. I remember reading how in ancient Ireland a battle took place in which sword blades were treated with a plant toxin (probably monkshood) so that even the slightest cut would be fatal.
But yes a blade soaked whilst cool is much more probable, dangerous thing though
