Гнида is not that rude not to be in the dictionary. That's the archaic word for louse, or any kind of parasitic insects, now it's used as an insult. It's not a real curse, though it's quite rude. Чернокожий means dark-skinned, this one isn't rude at all. Чернож***й (let me bleep it out) is a racist insult. Unless in English, in Russian the word черный, ''black,'' is a derogatory one. In Russian context, it usually refers to the people to the south from the European part of Russia, ie. Caucasus region, Caucasus countries, also Central Asian republics of the former Soviet Union and then down to the Middle East and Africa.

черножопая - black-ass literally

гнида - а nit чернокожая - black

This is a racist insult. Не брат ты мне, - you're not a brother to me гнида чернокожая! - a black-skinned nit.