I have seen it in person and on LG, MV lamps that get really old turn green. But why is this? It is still mercury inside. Does the arc tube blackening preferentially block out blue, turning the remaining light green? I am just guessing here. Does anybody know what is really behind this?

The arctube blackening blocking mainly the blue mercury line, resulting in a green light.
Also: If the lantern have a polycarbonate bowl, its yellowing turning the light green.

The blackening also absorbs the UV (it absorbs the shorter wavelenghts the most), so the red from the phosphor gets missing too.
So what remains are the green MV lines...

@dor123 @Medved
Wow! That was a complete guess on my part, I was not expecting that to actually be the case. What is so special about the arc tube blackening that makes it not absorb green but absorb the wavelengths shorter than green?

This kind of mess always absorbs the shorter wavelengths more than the longer, it absorbs the green too, but less than the blue and UV.
It is the same reason why a duststorm makes the sky yellowish brown.
The difference in MV is, the arc originally does not generate that much of red (to form the brown), but the red is created mainly in the phosphor from the UV.
So with the gradual onset of blackening, the lamp turns into higher CCT first when only the UV is mostly affected. Only later, when the darkening really progresses so it significantly reduces blue as well, the lamp turns green...

@Medved
That makes sense, thanks so much for explaining!

A small addition from the waste paper on this subject:


Source: W. Elenbaas and other, HIGH PRESSURE MERCURY VAPOUR LAMPS AND THEIR APPLICATIONS, PHILIPS TECHNICAL LIBRARY, 1965

Thank you for providing this invaluable resource, @Olav . 

Blackened arctube.