The photos were taken at around 20X magnification and the black bits aren't really visible to the naked eye.

The whole area is covered in residual flux and flux usually comes out of a syringe so it's clean, the black bits are something else entirely. Looking at the board they haven't reflowed using a hot air station so I'd guess it's a build up of burned crap that was on the soldering teezers or pencil iron they used. Maybe they scraped the shellac of the solder pads on the board using the iron or hot tweezers and that resulted in bits of blackened and burned varnish.

If it survives the testing regime I have planned for it and I end up keeping the tester I'll probably give the board an hour in the ultrasonic bath.