It wouldn’t surprise me if China had forty ‘black-prisons’ around the globe, and maybe even two or three in California.
You're probably right. I mean sure, the US border is porous in both directions.
But if they wanted to make someone disappear (for any one of the myriad of reasons tyrannical regimes do these kinds of things) the logistics would be a lot simpler to have a quiet little facility (or several) in-country. (ie. here) You'd want them in places where government officials were either complicit (ie. simply bought-off) or could be pressured to look the other way if/when the need arose. So of course CA, NYC, Chicago, heck even out in CO with the leftist wanna-bes.
It is likely there are several black sites scattered across the US. Not big, not noticeable. A small warehouse with a door big enough to pull a van or SUV inside and a few hundred square feet... Not residential, neighbors get curious. Not remote, activity stands out. Probably in light industrial use areas where no-one would take notice of semi-random comings and goings of work trucks and various personnel. Probably not used all the time. People would be detained, questioned, then disposed of...
How about the one in DC holding political prisoners whose only crime was going to a rally.