Cobalt is named after a goblin. Miners gave it that name long before anyone knew it was a metal at all.
In German folklore a Kobold was a household goblin, a sense recorded from the 13th century. Etymonline traces the word to Middle High German kobe, "hut, shed," plus a word for a goblin. That second part comes from hold, "gracious, friendly." Calling a troublesome spirit "friendly" was a way of keeping on its good side.
Silver miners in the Harz Mountains borrowed the goblin's name for a particular kind of rock. It contained arsenic and sulfur. According to the OED, as reported by Etymonline, the miners called it kobold because it degraded the ore and made them sick. Smelting such ore could release arsenic fumes, so the miners' complaint about their health was well founded.
The word moved into the learned writing of the 1500s. The physician Paracelsus mentioned cobalt as a troublesome, worthless mineral from mines on the border of Saxony and Bohemia. The mineralogist Georgius Agricola used it for ores that looked promising but gave no metal when smelted.
Yet the worthless rock had a use. In the mid-1500s Christoph Schürer found that it colored glass and pottery a deep blue, stronger than copper could. Etymonline notes that cobalt minerals were used for blue glass from the 16th century.
The metal itself was identified by the Swedish chemist Georg Brandt in the 1730s. Sources disagree on the year. Britannica gives 1730, Etymonline 1733, and the Elementymology reference about 1735. Britannica calls him the first person to discover a metal unknown in ancient times. Brandt showed the blue came from a new metal, and he kept the miners' name for it.
English has used cobalt since the 1680s, and "cobalt" as a name for a shade of blue since 1835. A neighboring metal got a similar name for a similar grudge: nickel.