I have been wondering about this, why is the blackest part of a fluorescent tube usually the side with the etch?

I would hazard a guess that this end is where the mercury dose capsule is, and it gets sputtered when firing the wire to release the mercury into the tube during final post evacuation first lighting.

OK.  Another thing is that the lamps in my avatar are the exact opposite of most tubes and looks like it is reversed where the blackening is on the end with no etch.  There is another pair that has basically no blackening and has been used the about same amount of time as these, though you can see the electrodes through the glass on one of the ends (also opposite of etch).

The tube blackening is of two kinds:
- Condensed mercury on the tube wall. This happen, when the mercury condense on the filament (quite frequent, as it is the first metal object in reach). During normal use this could happen with some designs on the other side than is the amalgam capsule, because with the amalgam capsule the mercury is drawn to that capsule, so does not go to the filament. When the filaments are heated for short time, the mercury is quickly boiled off and so settle onto cool tube wall nearby.
With designs, where the capsule is more intensely heated by the filament, such darkening happen around the capsule.
But all this mercury deposits are fully reversible and do not mean anything to lamp life. It could be recognized by the darkening disappearing as the tube heat up.

- Sputtered electrode material
With most designs the sputtered material is normally white, so does not cause the darkening, as it is the cathode coat. But when the cathode coat is consumed to a big extend and the naked tungsten wire become bombarded by ions, the tungsten sputter off and cause quite heavy blackening. With this the blackening occur first only on one tube end, it does not evaporate at warmup and it normally mean such side have quite large part of it's life over.
But sometimes the naked filament ends protruding through lead in wires catch the discharge (mainly on instant start, as these are the sharp end point to attract the discharge at first place) and as they are not coated, they sputter heavily (as their operating temperature is far above the normal cathode temperature). The thick lead in wire prevent the heat to spread to the coated part, so the discharge could stay there for some time. This sputtering does not directly affect the lamp life, as only the excessive part of the filament (what shouldn't be there at the first place, for sure not protrude so far it could reach the thermionic emission temperature). But the blackened tube end does not look good and it block some light output.