The BBC said Blair joined comedian Catherine Tate on Friday in a sketch set at his London office, where he became the target for her popular catchphrase "Am I bovvered?"
Afterwards, the prime minister joked about sharing the stage with Tate's schoolgirl character Lauren.
"Meeting Lauren was an experience in itself, she didn't seem 'bovvered' by anything," Blair joked. "But I hope she will put her time spent at Number 10 to good use."
While Comic Relief had been soliciting donations since Jan. 31, the BBC said that most of the $78 million was gathered during Friday's star-studded TV special.
The charity benefits Africa and British underprivileged.