Next week Monday is when he should appear again inside the catacombs. He appears only during Validation Week, currently it is Competition Week.
So make sure you are signed up to enter the catacombs (most likely you will want to sign up with Dawn) and next week the Black Smith will spawn randomly in a catacomb for a while, and then dissapear and spawn somewhere else. You will have to catch him in order to use him, and be sure to stock up on your ancient adena.
On Kain, the Hero chat often identifies the catacombs where the Blacksmith is. Turn Hero Chat 'on' using your Chat Window Option (the little hammer symbol at the lower left.)