From Chem4Kids.com:  http://www.chem4kids.com/files/elements/027_speak.html

The first thing most people think of when they hear the word cobalt is the color blue. While the purified metal is a grey steel color, the salts that include cobalt atoms are many shades of blue.

The element has been used for hundreds of years, but was named in 1735 by a chemist named Brandt. We often mention that you cannot find this element or that element free in nature. While cobalt is found in many minerals, it can be found as a pure element.

As miners have discovered, cobalt is often found in areas with high levels of arsenic (another element in the fourth period/row of the periodic table).