Well Shimano was in Japan making anodized aluminum parts and still mostly chroming their low end stuff except for the rare black skylark, and making Black DA in 1975 or so, as an alternative option to plain polished. Shimano still offers stuff in (bad) chrome on the low end, polished and anodized alumium, and painted. With the introduction of SIS across the Shimano lineup, you start to see painted accent pieces, not just in Sante. Whatever alternative theories there are, I'm thinking some people just liked black.
Steel parts are cheap to paint instead of plate, but quality steel parts are plated because paint wears down and exposes steel to rust at pivots much more easily. Paint also doesn't stick to chromed steel that well. It's not really surprising that after companies moved away from chromed steel on their high end products and aluminum was commonly adopted as a material that they started playing with adding color to parts to stand out, since it didn't need plating, anodizing can be colored, and paint adheres to aluminum well.