I am playing hardcore ultimate, but i havent even beat the core game yet. I was looking at my options from angrim just now and saw that he can craft the heart of darkness. I thought that was only supposed to be the hidden blacksmith later on.
I agree, the only way that can happen is in GD stash. You may have accidentally given yourself ALL blueprints and then saved it.
As @Nery says, go and visit your other blacksmiths, if they can craft HOD, then you have definitely saved from GD stash (or other tool) some effects you did not intend to.
i know for sure i dont have ALL blueprints, but i must have accidentally spawned that one. (when i decided to try hardcore mode, i gave myself a few blueprints because i dont have the time to grind for them…3 kids keep me pretty busy)