Which magician was said to have once lived at the Mansion where the Red Hot Chili Peppers recorded their album "Blood Sugar Sex Magik"?

Harry Houdini.

The band sought to record the album in an unconventional setting, believing it would enhance their creative output. Rubin suggested the mansion magician Harry Houdini allegedly once lived in, to which they agreed.

https://en.wikipedia.org/wiki/Blood_Sugar_Sex_Magik

"The Mansion" was originally built in 1918 in the Laurel Canyon region of Los Angeles, California.

It had been once owned by Errol Flynn and rumours suggest that Harry Houdini had also lived there. It has since been used as recording studios for musicians such as Audioslave, The Mars Volta, Slipknot, Linkin Park and the aforementioned Red Hot Chili Peppers.

https://wiki2.org/en/The_Mansion_(recording_studio)