Cobalt Bar

Cobalt Bars are bars crafted from 3 Cobalt Ore. Cobalt is the first tier of the three Hardmode ores that the player is able to obtain. Cobalt is the equivalent of Palladium. When the first Demon Altar is broken, Cobalt or Palladium is chosen randomly and the other one will not spawn naturally in that world.

On the, Cobalt Bars drop from Shadow Hammers.

Crafting all Cobalt equipment requires 143 /  145 bars, or  429 /  435 ores.

Trivia

 * Several Cobalt items have a distinctive Eastern theme, the spear being a naginata, the sword somewhat resembling a Katana, or the katana-like Muramasa, the Chinese themed Repeater, and the headgear variations resembling Japanese and/or Chinese headdresses.
 * Real isn't blue in pure form, but rather a lustrous, silver-gray. Cobalt oxide is used to make a, which probably inspired its appearance in Terraria. Historically, cobalt compounds were used for centuries to color glass, glazes, and ceramics in Egyptian sculpture, Persian jewelry, and Chinese porcelain (which may explain its Eastern theme in Terraria).
 * A Cobalt Drill appears to be mining Mythril Ore in the achievement "Extra Shiny!".