I drove my mom to see my niece's private school drama presentation at the Thousand Oaks Performing Arts Center (there are several schools which rent an auditorium there and put on a combined show). We were waiting outside beforehand in a paved area with ornamental plantings in containers, when a friend of my sister's felt something in her hair and brushed it off onto the pavement. It turned out to be this beetle.

The visible hindwings suggest it had been flying and just alighted before being thrown to the ground, and I don't remember what the plantings were anyway- so there's no association with plants to go on.

I lightened it a bit to show details- to the eye it seemed pure black in color.