AFAIK the "Dark Ages" are named like that because the Renaissance people thought they were enlightened, compared to the "barbaric people" who lived before them, not because we have less documents (although we do)
Perhaps originally, but the reason it persists as a term even when used by people who don't think medieval people were relatively unenlightened is that it can also be construed as referring to the lack of documents. (And for those who do regard medieval people as relatively unelightened, they might regard the lack of documents as something intrinsically associated with that.)
