According to this answer, new blacksmith plans are unlocked in New Game+. I know once you beat NG+, you move on to NG++, and so on.
What's the maximum "+" I need to be able to unlock all blacksmithing plans?
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NG+ is all you need. Beating the game any more than once is not neccessary to obtain all the equipment. I got every blueprint discovered before getting to NG++. If blueprints stop appearing in random chests, you need to look for rooms with 2 tougher versions of normal enemies and defeat them for a special chest to appear in the middle of the room which usually has the higher level blueprints. For examples, there's a room with 2 warlocks dressed in black named Amon and Barbatos. There's another room with 2 huge yellow skeletons. There's also one room with a large lance knight named Botis. Etc.
From stacking 5 curse runes before going to NG+, I was able to get all the armor except for:
-all armor in the last column
-2 pieces in the second to last column. I keep farming but am not able to get any other armor pieces.
I don't think one could get all that armor without stacking curse rooms though before going to NG+.