Solution below (Don't feel bad, I got completely stuck)
1) Use the sussur bark to change the colour of the forge fire in the blighted village basement
2) Put the selected weapon in and combine
I have tried this many times and I get no results. When I add the sussur bark, the forge will not turn blue. I have and have read the Journal and the blueprints. I have the sussur bark. I have the weapons. I have ignitedthe forge and fanned it with the bellows. I have followed 3 youtube videos ( all pretty much the same, of course) and I get no blue forge. Is this a glitch?