Apparently, in Dishonored 2, if you give the Black Market the Roseburrow Prototype you're supposed to get the Configurable Grooving blueprint for the crossbow.
However, I did not get this and instead got a discount.
Is that normal? I am really confused because apparently I was supposed to get a blueprint on completion of that quest.

If memory serves, the game spawns some items on the counter in front of you, and they're easy to overlook if you weren't expecting that and turn away to leave.

Bah, that must have been it . Silly design choice then, as you'd expect the blueprint to immediately be added to your inventory after mission complete.