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...)
Outside the walls was a different story. One day the Persians ran the place. The next day it was Arabs. China also fell, and whatever remained of the Silk Road disappeared until the Mongol Conquest.