I have noticed, especially on older fluorescent lights, the blackening is heavier on the non-etch end than the end with the etch (if it blackens at all). Does fluorescent blackening and etch have something related? thanks

All depends on which side is the "hot" side of the ballast. Etch placement has nothing to do with it.

I once put two lamps in opposite direction, in a dual lamp fixture. In one of them the etch side became darker, while on the other it was the non-etch side. It just depends on how you put the lamp.Newer lamps may have different filament resistance, so may not find the difference between the ballast's sides.

All depends on which side is the "hot" side of the ballast. Etch placement has nothing to do with it.

Well, except the modern T5HE/HO tubes, there the etch signalizes the end with the amalgam capsule (and longer "dead end" where the filament is further away from the seal), so in order to insert the tube with correct orientation, yu should follow the instructions on on which side the etch should be.

On magnetic rapid start, wasn't it hypothesized the yellow-wire side always aged first?

I doubt there would be any aging difference between "hot" vs "cold" side of the ballast at the frequencies the tubes use to operate (max few 10's kHz), even less on mains frequency.
What could make some difference is some difference in the heater power the ballast delivers (due to some assymetry).
It is true, longer wires on cold side (that is how the wiring is generally recommended), could cause extra wiring inductance in the longer "cold end" filament circuit, but unless some moron creates some huge loop from the wires (messy wiring, not keeping the wires routed alongside each other), the difference should be practically nonexistent.