What is the origin of the black arm band worn in some countries to indicate mourning?

The custom of wearing unadorned black clothing for mourning dates back at least to the Roman Empire, when the Toga pulla made of dark-colored wool was worn during periods of mourning.

Through the Middle Ages and Renaissance, distinctive mourning was worn for general as well as personal loss. After the Saint Bartholomew's Day Massacre of Huguenots in France, Elizabeth I of England and her court are said to have dressed in full mourning to receive the French Ambassador.
http://www.newworldencyclopedia.org/entry/mourning

Here's a source more specifically related to black armbands. It seems like the tradition of the black armband was started in England in the 1770s.

http://www.ehow.com/facts_7352356_history-black-armbands.html