fly in the buttermilk

noun2

1. •
   (informal) A black person in a predominantly white group of people.

   • You can do the same in parts of Ward Three and see black folks sprinkled like “flies in the buttermilk.” Two Districts of Columbia, two different realities. — 2008
2. •
   Synonym of fly in the ointment (“something that spoils something else”).

   • Discovery of this document was a fly in the buttermilk of Tattenai. — 2013