So first off - this outfit was meant to be a very dark grey rather than black.
That aside, you have to think what black looks like in the real world. English is not my first language so explaining very technical stuff is a bit difficult, but what it comes down to is even if an object is 'black' you have to think about the quality of the surface, whether light is shining on it, etc. I have a lot of objects that are black on my desk right now, but most of them look kind of dark grey more than anything, only the shiny objects look properly black. You have to imagine these outfits are made of a fabric similar to cotton, it's quite a rough fabric. I've got some darker fabrics in my wardrobe that are like this, and in comparison to something a little bit shinier, like silk, they look a lot lighter in the sun. The engine is trying to simulate all that, meaning that, realistically, you'll mostly see the type of black that you're looking for in the shadows. We actually have to be very careful that we don't make outfits even that have loads of different colours not too dark in shadowy areas because then you also get these strangely dark shadows there and everything ends up looking super unnatural. It's a real pain to balance everything but a fun challenge at the same time!
If you want to read more about the theory of some of this stuff with an explanation that's clearer than one I can ever give, check this out :) https://academy.substance3d.com/courses/the-pbr-guide-part-1 there's a part 2 as well.