You might be looking at the past without realizing that there is a foreshortening of time involved. The result of that foreshortening is to raise the apparent rate of important events. In your case, the important events are "good tunes," but it works as well with wars, deep philosophical insights, or whatever.
I'll try to explain how the foreshortening works. Imagine that weeks are like train cars, all strung together on a railroad track, near which you are standing. "This week" is the train car nearest you on the track. It's side faces you directly, and you are therefore able to see its whole length unforeshortened. Or almost.
But as you scan the weeks that have passed (in the direction toward which the train is moving), the train cars seem to bunch together. That's not only because of their distance, but also because of the acute angle from which you must observe them.
Likewise, events that happened in the past seem to bunch together into tighter bundles. It's as if all of 1967 had the same "angular footprint" in your consideration that the single day of January 7th, 2007, has.
So my thinking is that Blacks produced good tunes and utterly forgettable tunes in about the same proportion, back in the 1950s, as they do today. You've just forgotten the forgettable ones, and the foreshortening effect of your perspective in time makes the better Black tunes seem to clump more densely than they really do.