What does it mean to receive a black feather in the mail?

It means loss, illness, and death.You must have caused harm to someone. The black feather is an indicator that you will be receiving "threefold" in return until you make it right. The person that sent the feather seems to have very serious plans to get even and then some. These things really do work.Just dismissing them as silly will not do you any good.

Good Luck with that !