I'm trying to retrieve Gwent Cards, but cannot seem to find this one "Blacksmith" from "Silverton". He's nowhere to be found.
While searching for a solution, I came across this guide that lists the locations of all Gwent players in The Witcher 3.
I believe you're looking for this blacksmith:
The blacksmith in Silverton, north-west of Hierarch Square
Thus, your blacksmith is northwest of Hierarch Square, in Novigrad. Here is a map of Novigrad:
In addition, from this site, you can also see the locations of different merchants located in Novigrad and I believe the blacksmith you're looking for is labeled "Blacksmith (1)".