That moment when a person realizes they have deep, romantic feelings for a very close friend. A person's blackbird moment usually happens a significant period of time after their friend has expressed romantic interest in them. It is unexpected, sudden, and overwhelming.
The name "blackbird moment" comes from Glee's Blaine Anderson's blackbird moment, when in March 2011, he experienced this feeling for best friend Kurt Hummel, after Hummel performed a rendition of the Beatles' song "Blackbird".
Guy 1: omg I'm totally in love with my best friend.
Guy 2: omg blackbird moment.