This very old and famous surname, equally popular in Scotland and England, has at least two possible origins, the first being a nickname given by the invading Angles and Saxons to the native Celts and Britons who were darker-haired and darker-skinned than themselves. There is an ancient fable that Wulfricus Niger, otherwise known as Wulfric the Black circa 980, received his name after blackening his face in order to pass undetected through his enemies. The second possible origin is as a shortened form of Black-Smith, a worker in cold metals, as distinct from a White (Smith), one who worked in hot metals.

http://www.surnamedb.com/surname.aspx?name=Black

This is an international occupational surname. It can be said to originate from the Old High German (pre 7th century) word 'decken' meaning a covering, and as such in its true original form described a thatcher or roofer, or possibly a manufacturer of blankets and matting. In modern German the word 'decke' has the twin meaning of either a ceiling or a blanket according to the sense in which it is used.

http://www.surnamedb.com/surname.aspx?name=Decker