Historical figures go in and out of fashion like anything else-- even Jefferson has done so (and has his own detractors in the current political climate). Few people are capable of looking at History objectively, let alone from a contemporaneous point of view. Human History is a tapestry of exploitation and subjugation that we are still trying to evolve beyond, and we are far from finished with that process. Columbus's voyages came at the tail end of the Dark Ages, and in the context of his culture resulted in the discovery of a new world, which began an age of migration and colonization that changed Human History beyond measure. To regard him as either hero or villain is both simplistic and irrelevant to his Historical status.
It sure beats saying that he discovered America or that he proved the Earth is round, which is what was popular to teach american children for many years.
"Dark Ages" is far more evocative than "Middle Ages." "Dark Ages" sounds like knights and feudal castles and damsels in distress and evil kings; "Middle Ages" sounds like sitting on the couch and drinking beer while watching Wheel Of Fortune. But substitute "Middle Ages" if you prefer. 


