My understanding is that you have a pigment layer in your retina that carries light through the retina and
to the optic nerve. When I get my Central serous retinopathy
, the centre section develops something similar to a blister. It disrupts the flow of this pigment, and makes it visible, especially when I blink or move my eyes from a dark area or a light area. Basically, the blistered area can;t keep up with the non-blistered area and so the dark or light lingers in the area of disruption.