Reddit reviews Medieval Islamic swords and swordmaking (Gibb Memorial Trust Arabic Studies)

Treated with acid, yes. Don't know about colours. Here is an early Medieval example. Some discussion of this sword can be seen here. There are also pre-Medieval (e.g., Celtic) examples that appear to have been etched.

I don't know of any stained examples. Some blades were made with steel/iron with varying phosphorus content, which would give enhanced contrast similar to that obtained with nickel alloys. The right choice of acid would stain as well as etch.

Medieval Islamic swords were often etched/stained. The process is described by al-Kindi. I don't know of any Medieval examples with the original surface intact.

I don't know what the bluish modern Pakistani/Indian 15N20 blades are stained with. Indonesian keris blades are traditionally stained with an arsenic compound.

My wife got me Medieval Islamic Swords and Sword Making for valentine's day a couple years back, and it is fantastic. It's mostly primary sources, with some historical and metallurgical commentary where appropriate

I highly recommend it