The theme of the tea dance was Africa and many of the (almost exclusively white) participants came dressed in African attire with a few turning up dressed in colonial dress. What was judged worst of all by CNN was that some of the participants had even blacked up.
Although blacking up or blackface was acceptable in the USA and the UK as late as the 1970’s attitudes toward it have changed and it is now considered to be highly offensive.
CNN reports that the incident is particularly awkward for the museum (that did not organise the event itself) as it has in the past been accused of promoting an outdated colonialist view of Africa. The museum re-opened last year after a five-year renovation.