> Religious fanaticism I won't bother talking about
How would you characterize things like Children's crusades (and Crusades in general), Holy Inquisition, hard prosecution of every other religion and even other Christian fractions like e.g. Cathars?
And let's keep Byzantine out of this, I was talking about West specifically. West which experienced a significant population decline, wars, plagues, and which between 1st and 10th century produced very little of anything new. No big science discoveries, very limited architecture pieces - almost all churches, the (single) religion becoming the absolute focus of any artistic and cultural life. The remaining books and educated people were forced into self-censorship, as many philosophical ideas were considered as blasphemes, and Church positioned itself as one and only gatekeeper to knowledge.
If it was like you're claiming then West Europeans wouldn't need Arabic books to learn about ancient Greece achievements (as they had), and there wouldn't be an explosion of culture and education and book production as it was after fall of Cordoba and Toledo back into the hands of Christians. Arabic library of Cordoba is claimed to have had 400 thousands books, which was estimated to be more than what the whole Western Europe had at the time. In just a few decades the number of books in Europe multiplied many times, first Universities started to be created, things started moving. Just look at the dates when the first Universities were created in Europe: Bologna and Oxford 11th century, Paris and Salamanca 12th century, etc. It's not coincidence, they regained access to the Ancient sources that were lost before and that put them back in the saddle. If all the knowledge was preserved and readily available all the time as you've claimed, this peak would never be that drastic. And also there wouldn't be "a lack of historical records" if people were writing down the facts more - instead of concentrating on religious discussions.
And yes, Francesco Petrarca was the first known to use the term Dark Ages, and he specifically meant that because of the lack of great works and authors of the Classical period. But he wasn't talking about "the fewer historical records" - he freaking lived in that time and was talking about his own feelings that the culture of his time sucks. And great that he did, because he helped change that later.
And the term certainly wasn't used only by him, and not only in that context.