I ‘believe’ this is a red legged earth mite and not a blue oat mite. The digestive tract empties from the posterior and not the dorsal surface (back). Also has strong eye spots.
I measured its body length to be 1/2 mm. Being black, it is just visible bare-eyed (zipping around) on a white surface.
It has 8 legs and so is not a larva like a chigger, but a small nymph. It uses the first pair of legs as feelers and so walks on 6 like a chigger. It is the same size as a chigger and is closely related so a good window into how they get around on our bodies.