I appreciate comments saying that someone's race is not something you should put on as a costume when depicting characters they played - that in and of itself is a good point, and an interesting one.
But I feel that simply calling what the woman did "blackface" fails to really connect most people to some of the wounds that are at play here.
Largely because current/younger generations, especially white people, have little-to-no awareness or concept about how pervasive blackface minstrelcy was, to what purpose it was used, and how it has continued to influence how characters of color are depicted in theater and the media, and also how people of color are perceived on the street.
If you are confused by the level of emotion invested in the backlash directed at this white cosplayer, I encourage you to read Let Black Kids Just Be Kids, a recent and very relevant New York Times Op-Ed by Robin Bernstein.
Berstein draws a very credible connection between the practice of blackface minstrelcy and the current patterns of perception that contribute to tragedies like the death of Trayvon Martin and the daily overpolicing and underprotecting of black kids in America.
Bernstein goes on to explain, "only white kids were allowed to be innocent. The more that popular writers, playwrights, actors and visual artists created images of innocent white children, the more they depicted children of color, especially black children, as unconstrained imps. Over time, this resulted in them being defined as nonchildren."
“Uncle Tom’s Cabin,” one of the most influential books of the 19th century, was pivotal to this process. When Harriet Beecher Stowe published her novel in 1852, she created the angelic white Eva, who contrasted with Topsy, the mischievous black girl.
Stowe carefully showed, however, that Topsy was at heart an innocent child who misbehaved because she had been traumatized, “hardened,” by slavery’s violence. Topsy’s bad behavior implicated slavery, not her or black children in general.
The novel’s success prompted theatrical troupes across the country to adapt “Uncle Tom’s Cabin” into what became one of the most popular stage shows of all time. But to attract the biggest audiences, these productions combined Stowe’s story with the era’s other hugely popular entertainment: minstrelsy.
Topsys onstage, often played by white women in blackface, were adultlike, cartoonish characters who laughed as they were beaten, and who invited audiences to laugh, too. In these shows, Topsy’s innocence and vulnerability vanished. The violence that Stowe condemned became a source of delight for white theater audiences.
This minstrel version of Topsy turned into the pickaninny, one of the most damaging racist images ever created. This dehumanized black juvenile character was comically impervious to pain and never needed protection or tenderness.
Other Blackface Minstrel characters created or perpetuated equally damaging stereotypes of black people, almost all of which were used only as comic relief, and young white people today are largely ignorant about those characters and performances, which continued into the 1970s in some places.
Heck, most of us don't even know or think about where the term "Jim Crow" came from.
Sometime around 1830, a white NYC ctor named Thomas D. Rice learned a popular African-American song-and-dance routine, based on the myth of a trickster figure, an escaped, possibly physically disabled slave named Jim Crow, who would dance and boast. His face blacked out with burnt cork, Rice perfected the act and sparked the tradition of the minstrel act. At first the blackface character was actually a smart and sympathetic one. But as time went on, the minstrel show took on a more racist tone.
Blackface Minstrelcy was very popular in the US in the 1830s and 1840s, but it continued to be practiced well into the 1900s and its legacy continues in the lack of African American characters of content who have primary roles in mainstream US media (books, film, plays, tv shows, etc). This legacy has caused an implicit bias in almost all of us, too little acknowledged, and is very hard to address with counter-cultural re-programming, especially while white people continue to dominate the production fields of those media.
If you think people are over-reacting to white people who darken their faces to portray people of color, please take a step back and learn more about why they are reacting the way they are.
On the topic of depictions in film and theater, btw, I highly recommend you read the historical fantasy novel Redwood and Wildfire, by playwright Andrea Hairston.