Focusing on the Craft:
* Forge
* Anvil
* Hammering
* Clang (referencing the sound of hammering)
* Strike
* Ironmongers
Focusing on the Group:
* Guild
* Company
* Crew
* Band (playing on the idea of a band of brothers)
* Assembly
Humorous Options:
* Shoeing Party
* Sparks
* Hotheads
* Iron Will
You can also use more general collective nouns like group, set, or collection for a blacksmith flock.
Ultimately, the best collective noun depends on the specific context and the tone you want to convey!