Any hammer can be used for Blacksmithing! However, like any hobby or profession there are always tools that make the job faster and easier. The hammer for blacksmithing is not an exception. Hammers are all similar in that they all have a handle and a head, but there are a number of different characteristics of each that affect the ability of the hammer to accomplish the task at hand.
Blacksmith hammers as a rule are heavier than your typical carpenters hammer. The added weight make it easier to apply great force upon the metal you are trying to shape. Handles vary in composition and length. What you prefer is a personal preference so lets concentrate on the head.
Hammers that blacksmiths use range from the small sub one pound ball peen hammers to heavy sledge hammers for large thick metal jobs. The typical blacksmith hammer is in between these two extremes, usually 2-3 pounds (1-1.4 kg).
The other big difference between a blacksmith hammer is the backside of the hammer. Instead of a claw you usually find something more like a hatchet called a cross peen hammer. This hammer is however, not sharp like a typical hatchet, but instead is more rounded dull by design. The cross peen hammer forces the metal to spread in two opposite directions when the metal is struck. Whereas, the normal face side of the hammer spreads the metal in all directions equally.
A less common hammer is the straight peen hammer. The only difference between a straight and a cross peen hammer is that the cross peen hammer has its cutting-edge running side to side, whereas, a straight peen hammer has its cutting-edge runing up and down which is inline with the handle. Sometimes you can even find peen hammers that have the peen running at 45 degree angles.
Another type of hammer used by blacksmiths is the ball peen hammer. On this hammer the claw is replaced with a rounded end. A blow from the a ball peen hammer works much like the flat face of the typical hammer but since it is not flat but instead rounded it does a better job of spreading out the metal.