I've seen blacksmithing plans spawn in five different places in BRD, but only in one of them each run, and I have had runs where I didn't see any at all and suspect there may be more spawn points. When they do spawn, they have one of several different plans, the Dark Iron plans that Lokhtos Darkbargainer does not sell - Dark Iron Plate, Dark Iron Shoulders, Dark Iron Mail, and I believe the weapon plans Dark Iron Pulverizer and Dark Iron Sunderer - possibly others. The five spawn points I have seen are:
1. On the upper level of the Shadowforge lock room - if you are going up from the lower level, they are on one of the shelves in the corner of your room to the left at the top of the stairs. If you are coming up from the ring of law, they are in the corner diagonally to your right.
2. In the manufactory - there is a small workbench to the right of the opening which leads to the Grim Guzzler which sometimes has the plans.
3. Also in the manufactory, the plans are sometimes found on the shelf just to the right of the opening into Golem Lord Argelmach's room.
4. In the room with General Angerforge, as Bikuti said, they can spawn on the shelves in the corner with the three humanoids opposite the side the stairs come down.
5. In the large room of Golems and Arcanasmiths(?) just beyond Ambassador Flamelash and just before the seven ghosts room, they can spawn in the corner to the right at the base of the ramp as you enter the lower part of the room.
It never occurred to me that each separate spawn point may have a unique pattern - if possible, that would explain why you always find the same pattern in the spawn that you find next to Argelmach's room.
I've never noticed plans where FenrirSulis puts them, in the small anteroom between the stairs to General Angerforge and the ramp down to the Manufactory - if it's there, that would be a sixth spawn point.
Any others?
Edited, Aug 3rd 2006 at 2:33pm EDT by fledarmus