I read about this a lot a couple of years ago, and your question prompted me to revisit the book "Catastrophe". In this book, there was an explosion as far as the year 535 AD which maintained the adjacent land in ash-caused darkness, affected the crops, and activated a series of spreadable illnesses not to mention those caused by the atmosphere itself. This eruption was evidenced by the trees in the area, whose rings showed massive commotion and darkening dating back to those days.
The 1883 explosion left the area in complete darkness for three MONTHS! It equally affected the quality of life around the area, and its secondary effects reached Greenland.
Could Krakatoa be responsible for the dark ages? Well, it did send civilization backwards and did not help it progress for a long time, but I would consider it one of MANY factors that perpetuated the Dark Ages- there is much, much more to THAT story.
I am including a site that could help you build an even more informed decision on how you feel it may have affected these historical periods. Good luck!