An excerpt from http://en.wikipedia.org/wiki/Blackface

"Blackface is a style of theatrical makeup that originated in the United States, used to affect the countenance of an iconic, racist American archetype — that of the darky or coon. Blackface also refers to a genre of musical and comedic theatrical presentation in which blackface makeup is worn. White blackface performers in the past used burnt cork and later greasepaint or shoe polish to blacken their skin and exaggerate their lips, often wearing woolly wigs, gloves, tailcoats, or ragged clothes to complete the transformation. Later, black artists also performed in blackface.

Blackface was an important performance tradition in the American theater for over 100 years and was also popular overseas. Stereotypes embodied in the stock characters of blackface minstrelsy played a significant role in cementing and proliferating racist images, attitudes and perceptions worldwide. In some quarters, the caricatures that were the legacy of blackface persist to the present day and are a cause of ongoing controversy.

By the mid-20th century, changing attitudes about race and racism effectively ended the prominence of blackface performance in the U.S. and elsewhere. However, it remains in relatively limited use as a theatrical device, mostly outside the U.S., and is more commonly used today as edgy social commentary or satire. Perhaps the most enduring effect of blackface is the precedent it established in the introduction of African American culture to an international audience, albeit through a distorted lens. Blackface minstrelsy's groundbreaking appropriation, exploitation, and assimilation of African-American culture — as well as the inter-ethnic artistic collaborations that stemmed from it — were but a prologue to the lucrative packaging, marketing, and dissemination of African-American cultural expression and its myriad derivative forms in today's world popular culture".