Negrosity is a term used to describe how black someone or something is. Once someone's Negrosity Levels rise to a certain level they may often start to radiate Negrotism.
Johnny smoked a Jeffrey and his Negrosity levels instantly rose so high that he started to radiate Negrotism.
Kind of the same as necrosis, only your skin turns black, your lips get big, and you have a sudden craving for chicken and watermelon... only it still rots you from the inside.