By default, when you first get the Ghost Armor in Ghost of Tsushima, it will be sort of grey mostly, but it comes a bit close to Black when you fully upgrade. But if you want something like solid black even in sunlight, then here is how you can get it.
Find a Black Dye Merchant, he will be inside a cave, here is the location:
It’ll near Kawamata Village as shown in the map above.
From him, you can get the “Black” color look for all the armors and even your bow. Get the “Shadow of Justice” skin from him. It’ll just like this, but I changed the headband and mask. I loved this look.
That’s all. This one looks DARK BLACK compared to the default one. Do let us know if you have any queries!