User blog comment:Fairy Tail Grand Master/What is the hidden blade made of?/@comment-4564231-20120629090026

Folded steel seems like the most logical explanation though I'd say it may contain traces of some rare mineral (because Altair said the materials were rare to come by so the Assassins would have to limit the number of recruits that were allowed Dual Hidden Blades). It may also be some new alloy using a common metal like iron and some other uncommon mineral/metal. I'd suggest you get the hidden blade+bracer replica they're selling and run a few tests on it.