I've seen these mites before, and like yours only in queens of C. castaneus. They may belong to the genus Oplitus. I never saw more than two mites per queen; they always seemed to attach to where the tibia meets the tarsus on only the third pair of legs. They're likely specialized parasites on this sole species, and on this specific body part, which would explain their color. I doubt they'd reach harmful levels, and with my queens I never saw the mites again past the first workers. If you want you can cool the queen down and pick them off carefully with tweezers. Or, just don't worry about it too much.