The word itself derives simply from being a "smith" and working with "black" which referred to black iron, as opposed to goldsmiths or silversmiths. (A goldsmith was such a high-profile profession that the surname Goldschmidt still exists.)
Blacksmiths needed a variety of tools: an anvil, a hammer, and a set of tongs at the very least. They also developed other tools for shaping metal. They also needed a very hot fire. Iron had a very high melting point, so the smith's goal was to heat the iron sufficiently to make it "plastic" so it could be hammered into shape. This was not a quick process, so required endurance and strength.
The heating required charcoal—a lot of it—which had a benefit that was noted if not completely understood. The repeated thrusting of raw iron into the fire to keep it malleable produced a stronger and more durable metal object. The smith might have seen the outcome as the result of more repeated hammerings, but the truth was understood only much later: greater exposure to the hot charcoal added carbon (perhaps only from 0.5-2%) to the iron, as well as removing oxygen, making it stronger and less likely to crack or rust. This improved daggers and swords and armor.
Of course, it wasn't all about armor and weapons. Blacksmiths made tools, door hinges, axles for carts, hooks and hangers and sconces, pots and pans, locks and keys, horseshoes and harness, and any of the myriad items needed in daily life that could not be efficient if made of wood.
Men were not the only smiths. In 1346, Edward III of England appointed Katherine Le Fevre to "keep up the king’s forge within the Tower and carry on [its] work … receiving the wages pertaining to the office." In the early 1300s, an Alice la Haubergere worked in Cheapside in London, making armor. In York in 1403, Agnes Hecche inherited her father's chainmail equipment after he died, and continued the family business.