There was an interesting article about Roemer and his measurement of light in the American Journal of Physics about 20 years ago or so. It surveyed several books giving accounts of his work, and found that they gave vastly different results. Then they went back to Roemer’s original work and, they claimed, found that Roemer hadn’t actually calculated a value at all! (I find this hard to believe – I figure that he must at least have figured a range of values, but I haven’t gone back to look through Roemer’s work). In any event, they claimed that assertions about Roemer’s value are all untrustworthy.
Roemer, at the very least, proved that light has a finite velocity (he did it by measuring the time lag between the expected appearance of Jupiter’s satellites and the actual appearance), something that had been in question for a while, at least since Galileo’s attempt to measiure the speed of light between two mountains failed. AFAIK, the undisputed values for measuring the speed of light first occurred in the 19th century, with experiments like Fizeau’s, using rotating mirrors.
(We had students duplicate this experiment for an optics class I taught)
As far as the “speed of dark”, there was once a hack article in an optics journal about the “darkon theory of light”, which suggested that there are actually particles of negative energy, called “darkons”, that carry dark. You can measure the speed of darkons by measuring the rate of travel of an interruption in a row of darkons, which can be called a “photon” by the uninformed. So the speed of a darkon is the same as the speed of light.