Just because the boundaries are fuzzy doesn't necessarily invalidate the concept of a 'dark age'.
If you don't like the term, we could call it "the age where total population, urbanization levels, manuscript production, trade volume, lead pollution, [placeholder for other convenient or inconvenient metric] in Europe[1] stayed far below peak Roman levels" - but that's kind of a mouthful...
[1] 'western' Europe in particular, but some of the effects are visible at continental scale, or even globally
If you don't like the term, we could call it "the age where total population, urbanization levels, manuscript production, trade volume, lead pollution, [placeholder for other convenient or inconvenient metric] in Europe[1] stayed far below peak Roman levels" - but that's kind of a mouthful...
[1] 'western' Europe in particular, but some of the effects are visible at continental scale, or even globally