Not everyone blacks up, Les. Leicester Morris "red up" on Plough Monday to become Red Leicester Border. It hearkens back to the use of reddle or ruddle to mark tups. The soil round Leicestershire and Rutland is clay-rich (Ruddle-land = Rutland).

I don't think this could be construed by anyone as any kind of racial slur, but these things are all about context, and many colours can offend providing the context is right: the children at my daughter's primary school were once face-painted yellow to represent Chinese people...

Does Darkie Day have racist roots? Well there was a really interesting piece in EDS magazine a while back, which concluded that although blacking up is attributed by many who do it now to the need for disguise, there are hardly any references to morris dancers blacking up before the advent of minstrelsy...