So i've been playing around with the black assassin outfits and there are actually a few ways to get them. The fastest way is probably checking out izumi settsu, where you can find some really nice stealth gear in the legendary chests around katano castle. The swift shinobi hood is pretty sweet, slows down time when enemies spot you which has saved my neck more than once.

If you're looking for the full black setup though, you'll want to explore this hidden trail i found northeast of osaka. There's a chest there that usually drops a pure black hood with some nice perks, just make sure to save before opening it if you're hunting for specific bonuses like the kunai effect.

For the absolute best black assassin gear though, you'll need to finish the main story and epilogue missions first. The master assassin set you get at the end is honestly worth the wait, comes with this really nice red and black color scheme. While you're working towards that, definitely upgrade your gear at the hideout forge when you can, makes a huge difference especially for those tricky castle infiltrations. And if you're having trouble getting to some of the chests, smoke bombs are your best friend when dealing with those high-level guards, trust me on that one.