Nicotiana is very sticky. If you have a tree above it that's dropping a lot of debris it could just be sticking to the plant. Ckeck it out with a magnifying glass... see if your dirt had legs or moves around at all.
My Nicotiana sylvestris gets black aphids too - just covered with them. In a recent thread I said I only get black aphids on nasturtium but when I saw your picture it all came back to me! Never seems to stop them from blooming and setting seed tho...come to think of it, nothing seems to bother them.