It probably originates from British historians, who had no idea of what if anything existed between the withdrawals of the Legions, and the rationalisation of the Anglo-Saxon influx into nominate "kingdoms". They had heard rumours of King Arthur but lacking documentary proof he existed, they dismissed him. They had ecclesiastical works, most of which they dismissed as hagiography, collected pagan legend (all, of course, untrue) and milleniallist rantings. They had occasional chronicles, which were unsupported by "hard evidence" (such as earlier academics' learned disquisitions, or undemolished buildings).
They therefore constructed the "Dark Ages" as a period where Nothing Of Significance Happened, and people simply worked all day long and sat around in the dark at night, waiting for the glorious sunrise of academia, which was taken, pretty much, to commence with King Alfred, who as well as burning drop scones, set up (or tried to) a library. (Cnut obviously didn't count -- he did silly things with thrones and tides and anyway, he was Norse.) The historians' definition of Full Daylight probably coincided with Edward the Confessor, since they knew he was supposed to have been a learned man, and since they needed to discuss whether Harold Godwinson had or had not sworn holy oaths to William the Bastard prior to his (Harold's) seizure of the throne on Edward's death, and therefore whether William was a bloody invader, or simply a man coming to claim his rightful realm. (Since Hardrada died, he was damned as an invader either way.)