The color of a spindle isn't a reliable indicator of the type of steel from which it's made. Both Shimano and Hatta produced some CroMo spindles with a black finish. Here's one example, from Hatta:




And here's another example, this one from Shimano:





I suspect the shiny ones are plated. I've seen photos of higher-end shiny spindles (Dura Ace, if I remember correctly) where the plating is peeling at the taper.

OP: I strongly suspect that the smoothness of operation of a new cup and cone bottom bracket is more related to the smoothness of cup races and spindle cones, along with the grade of the bearing balls used. Higher end spindles tend to be better ground and polished, and also tend to be plated. The type and hardness of the steel used in the cups/spindle cones would almost certainly have far more impact on longevity than on smoothness of rotation.