A useful trait to check is to see if there are small pale patches as are commonly seen in Hippodamia moesta.



Because your specimen is so dark and worn/damaged, I think you'll need a decently bright light or flash to see this. I can make out the face patch, but it's hard to see. The wing tip pattern may require angling a light specifically so that it goes through it and doesn't bounce off the body. If there are pale patches, you should see them this way.

This beetle appears to be a Hippodamia based on the body profile and structural details like antennae. The body is narrower and more oval than Hippodamia, and flatter when viewed from the side. There happens to be an all-dark species found specifically in this part of the coastal western US. The underside colouration that is mostly dark with a yellowish tip is also very suggestive.

This is not a common beetle either way -- black forms of Harmonia are possible, and I've seen one in person, but they are rare.