There is no doubt that the sap of upas tree is very poisonous. I think that they used to give condemned criminals the job of going to collect it, as an alternative to execution. It was used as an arrow poison. I've just googleised "upas tree" and there's quite a bit there on it, which seems to more or less agree with what I can recall.
There is a poisonous frog in Jawa, but I don't know how poisonous, or if its poison was ever used on keris.
I personally think that it is possible that sometimes, some people may have applied poison to a keris blade before they entered a situation where they knew they would probably use the keris, but I have never heard of this as a general practice.
We can forget the arsenic as a blade poison. Arsenic does not work that way, and the tiny quantity that might be left on any blade would never do any damage anyway.
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