In 1986, Adrian Piper made small cards with the following text printed on them. She handed them around when necessary. Here, find a copy-and-pastable version of the message. Feel free to use in emails, comment sections, private messages or whenever you feel the need.
Dear Friend,
  I am black.
  I am sure you did not realize this when you made/laughed at/agreed with that racist remark. In the past, I have attempted to alert white people to my racial identity in advance. Unfortunately, this invariably causes them to react to me as pushy, manipulative, or socially inappropriate. Therefore, my policy is to assume that white people do not make theses remarks, even when they believe there are no black people present, and to copy/paste this message when they do.
  I regret any discomfort my presence is causing you, just as I am sure you regret the discomfort your racism is causing me.