The Blacksmith Trophy in A Plague Tale: Innocence
Find Rodric's forge
How to unlock The Blacksmith~If missed, can be obtained via chapter select~
Chapter 16: Coronation
As you make your way towards the cathedral, you'll head through a burning building. Once you're out, you'll save two captives near a cart. The forge you're looking for is nearby.
To progress the story, you need to push the cart forward so do not start pushing it yet. Instead, look for the forge first. There is an alcove with a workbench next to the cart. As you're facing it, turn around and you'll see an alleyway on the left. Go through it and turn right at the end. You should have a clear shot at the lock of a gate. Rodric will say "This place, I know it all too well". It'll be obvious you are on the right track. Destroy the lock with your sling and go back around to where the cart is and go through the now unlocked gate. Run all the way to the back of the road to find the forge.
Video Guide: LINK
Credit to NonstopTrophies