I recently saw some MV fixtures, used 400w H33's, and coated, totally as dim as a 25w incandescent lamp, (sorry, didn't have camera w/me), and don't know why this happens. Does the arc tube pressure decrease or something?

The arc tube blackens as the electrodes sputter and as a result the light output is reduced. Furthermore, the phosphor coating wears out, decreasing even more the light output.

I have seen Mv with very blacken arctube and it almost emit no light at all, but the ballast is humming away

The MV become first bluer and then green, because as the arctube inner coat of the sputtered electrodes thicken, it first block shorter wave radiation, so first the UV (what the phosphor uses to generate the red), then in the visible it continue with the blue part, so what remain are the green and yellow lines, what gives the final greenish appearance...