We have 70w MH recessed can lights where I work.  One of the lamps had failed and turned jet black.  It was a Sylvania, and I had thought it had an arc tube leak.  Anyway when they came the other day and relamped the fixture the new lamp failed to work, and instead it was the ballast that had died.  Is there any way that when the ballast died, it would have caused that black damage to the lamp?  I have never seen anything like it before. This light also stays on 24/7.

It is more likely to be the other way round - the lamp's behaviour at EOL killing the ballast.
A rectifying lamp can cause the ballast to severely heat up and in turn result in failure from internal short or blowing thermal fuse.
The lamp blackening could be either the result of a arc tube leak or even an explosion (this lamp maybe had an internal shield)

MH lamps should not be left to burn 24-7 or beyond their rated lifetime as they do horrible things at end of life - such as frying control gear and/or blowing up!

The lamp was a protected arc Sylvania, hmm never thought of the arc actually exploding, but the bulb was really black.  Anyway I work in my university library and they have about 50 MH can lights.  21 of those run 24/7.  I'm not sure how old some of them are, but a lot of them are dim and flicker, one even strobes.  Nobody around there cares, I know the other 3 ballasts that died was because of dead bulbs being left in there. I have worked there for over a year and those lights were out then!! 

I can see the headlines now....

'LIBRARY GUTTED IN LATE NIGHT INFERNO'

Firefighters were still damping down the smouldering remains of a library this morning after a huge fire totally engulfed the building in the early hours. Fortunately there was no-one there except a security guard who managed to escape.
Preliminary investigations suggest that an electrical fault in a poorly maintained lighting system is to blame for the blaze which caused $8million worth of damage. According to the security guard, a flickering light fixture suddenly went out with a loud 'BANG' and sent red hot fragments of glass flying down on to the books below which, being made from paper, ignited very quickly.

 

Metal Halides have a scary EOL!

Another possibility is igniter failure...leaving a dead lamp in powered fixture will cause igniter to run constant and might also be bad for ballast.

Metal halide downlights seem to be a poor choice for a library, although there seems to be a period of time when it was popular as I have seen MH in libraries several times before. It bothers me when fixtures aren't maintained well.

After relamping, did somebody to cycle the power? I ask, because many EOL protection is designed as ignitors with a timer, what stop's ignition attempts after some cumulative time. And to let them work again, you should turn OFF and then back ON the power to reset these timers...
And i think it would be better choice for such application to do group relamping at ~70..80% rated lifetime in the scope of regular maintenance, simply to avoid problems with lamp explosions and ballast damages...

in the king soopers parking lot (king soopers is the kroger of colorado or the west...)

there are 2 parking lot poles (with 2 fixtures on it) that are on 24-7. i wonder when they reach end of life, or since there on 24-7, it will blow up (since its MH) and i see an atricle on the news "Parking lot light blows up hitting cars, damaging cars, and injuring people" i can see a headline like that since those lights are on ALL the time.

The Stine Lumber here in is lit with 400w MH high bays.  Probably 30% of them are out with either bad bulbs, bad ballasts or both.  A few of them even have fragmented arc tubes.