Conflicting references to why the “Dark Ages,” are “Dark.” I first learned it was Dark because of superstition & the loss of the civilizing Roman Empire. Later, I learned “Dark” actually refers to a general lack of historical knowledge of the era. Which is correct?!

The term has been used historically to mean both of those. The term 'Dark Ages' is actually relatively new, it only gained prominence in the 19th century and it is somewhat unique to the English language, where it split in the early 20th century between the Dark Ages for Anglo-Saxon England and the Middle Ages for Norman England. (There are cognate terms in other languages, like 'finsteres Mittelalter' or 'secoli bui', but they don't seem to have quite the same relevance or meaning as 'Dark Ages' does in English.) I've written about the development of this terminology here.

There is a separate but related use of the term, as you note, to refer to a period from which we have very few sources. (I believe German distinguishes this terminologically with 'dunkles Jahrhunderte' as opposed to 'finsteres Mittelalter', but as a non-native speaker I can't say for sure.) This use is both more general and more specific than the previous. It is more general as it is applied to a range of periods, for example there is the so called Greek Dark Age, between the Mycenaean (ca. 1200-1100BC) and archaic periods (ca. 700BC), and Byzantine Dark Age, from after the reign of Justinian (d.565) to the beginning of the Macedonian Dynasty (811). But it is also more specific, since when applied to the Latin Middle Ages, it tends to refer specifically to the migration/Merovingian period (~5-8th century) or to the papacy in the 10-11th century. But while some scholars still use this terminology in reference to the Greeks and Byzantines, albeit normally with some qualification, it is essentially no longer used in reference to the Latin Middle Ages.