If you want to go dark, ferric chloride is likely going to be your best bet.
But you can get decent results with anything acidic, with enough time. I have a preference for slightly heated aple cider vinegar. I'm not sure why, but it gives a nice iridescent sheen with relatively brief soak times. Get it up up maybe 120ish F, soak about twenty, check to see where you are, then every fifteen or so after that until you get where you want to be.
I've not done a lot of d2 though, mostly stuff like 1095. D2 is a bit more resistant than your usual carbon steels, so I can't be sure how long it'll take tbh. It works on d2, but the degree/time is nebulous on my end.
