There's roughly an 80 year period where no new art works were produced in the City itself, and afterwards it is clear to anyone the Classical Period was over. Only inferior pieces came through the imperial studios. Michaelangelo, produced the first art to equal that of the Classical Period about 800 years later.
Still, the level of depopulation suffered in the Eastern Empire was much less than that of the Western Empire which suffered a total social, governmental and economic collapse.
No doubt Byzantium was in good shape compared to the West. On the other hand, the Persians had been so weakened that it was a trivial task for the Meccan and Petran Arabs to destroy their empire in a few years. Once they'd done that, and offered to pay renegade Byzantine armies (who hadn't been paid for several decades), it was 1, 2, 3, and the Islamic Caliphate began to take shape.
BTW, the City of Byzantium was supposed to have sufficient resources for a 7 year siege, and that indicates that the climatic catastrophe that brought on the Dark Ages only lasted a few years ~ most observers view it as something on the order of a rather widespread and lengthy Fimbul Winter.
You can continue to believe the Dark Ages didn't happen in the Eastern Empire, of course, but you have to explain the decline of the Persians and the rise of Mecca ~ which, for gosh sakes, is still a small city ~
Justinian's Idiocy (the reconquest of Italy) broke the byzantine's land army, ate up its navy, emptied its coffers, and helped transmit the plague.
It was very bad luck that Justinian's Idiocy coincided with a large eruption on the other side of the world which caused a drop in european temperatures significant enough to set off the plague.
It was *extremely* bad POLICY that, in support of his Idiocy, Justinian oppressed the Levantine, Egyptian, and Arabian Monophysites - making conversion to Islam a small step.
And, of course, the usual biff-baffing between Khosroes and Justinian surely didn't help the strenght of their respective empires in Syria and Mesopotamia.
yes, byzantium had hard times, but they did not fall into a dark age (total collapse of culture, nation, government, economy, trade, alliances, knowledge base, arts, etc...)