Of course it's a mental illness...

Here are two examples of labels of mental illness applied to slaves (and
free black-skinned people in the time when slavery existed)
http://en.wikipedia.org/wiki/Dysaethesia_Aethiopica
and the famous
http://en.wikipedia.org/wiki/Drapetomania

It is immoral how the label of mental illness is used on people with
black skin even in modern Britain.
http://www.iop.kcl.ac.uk/departments/?locator=398&context=997#variation