In short order, she will be painted black with the same brush all of your painted Martin Gibson with: the one that paints everyone into the boxes that easily and conveniently fit inside the dark box of your preconceived notions most here refuse to admit to having"

From Websters:   

"black: ... 6 a :thoroughly sinister or evil : WICKED (a black deed :indicative of condemnation or discredit (got a black mark for being late)"

"dark: ... 3 a : arising from or showing evil traits or desires : EVIL (the dark powers that lead to war) b : DISMAL, GLOOMY (had a dark view of the future)"

Of all the possible definitions/usages of the words "black" and "dark", the ones I posted are the only 2 that fit with the context of GUEST'S comment.   Won't speak for GUEST of course, but I bet no racial slurs were intended!