So I'm now convinced that some time of particular structure is needed to breed a blacksmith:
I have 4 different "villages" that I created from scratch. Each one consists of nothing more than a single room with several wooden doors. Then I also have 2 actual villages that were created during world generation.
In the 4 villages that I created from scratch, I've gone through about 40 rounds of villager breeding (in each one). Not a single time resulted in a blacksmith.
In the 2 villages created during world generation, however, after about 5-6 rounds of villager breeding, I was able to produce a blacksmith.
Is there a special structure needed in the "village" to produce a blacksmith? Is the blacksmith structure (furnace, lava with iron bars, etc.) actually needed?