I have a question about some MH lights.  The fixtures are recessed can lights and I'm guessing the wattage to be 70 to 100 watts.  The fixtures are enclosed and have a clear piece of class over the bottom of the fixture. Anyway, 4 of them burned out and when replaced I noticed that the old bulbs were completely black (they were coated bulbs)  First of all, could this be caused by the bulbs being used in a closed recessed light??  I know people have mentioned arc tube leakage, but I find it hard to believe that all of the bulbs would behave that way.
Second question, two additional fixture tun very dim and flicker excessively.  What is the cause of this??  Does running the lamps in this condition pose any arc tube rupture risk?
Last question, I noticed yet another old bulb running and the end of the bulb was black.  The bulb was running so dimly that I couldn't tell if the bulb was swollen looking or just because the end was black. Anyway, after seeing it this way I'm afraid that this bulb might burst, but at least the fixture has a glass cover on it.

Good question Bruce.....I do know a few things about MH...but not enough.....I will let the others know and answer your question.....I know some others that knows.....also someone here might know and answer your question.....

I do know that some exploded lamps can turn black too! And the flickering one has unstable arc but not sure the specific reasons...but I know about somethings such as "salts" and stuff

Interesting situation! Seems that enclosed lamps do run hotter, and that could degrade phosphors more, but not blacken them. The first thing that comes to mind for me is the possibility that there was a bad batch of lamps. I understand that some Sylvania's had problems with leakage for a while. Then they redesigned the smaller sized bulb arc-tubes....like the 70 watters, for example.

On your second question, were those dim lamps replacements?

On the last question, yes, a blackened bulb can increase the chance of rupture since the blackness will cause it to retain more heat, especially if it is in the arc-tube. A hotter arc-tube can weaken since they operate under high pressure. As for the outer bulb being darkened, I doubt the outer will swell, but the arc-tube certainly can. I'd like to hear other replies about this.

I'm not sure what brand of bulbs these were, but I do know that the replacements were Sylvania's.  No, the dim lamps were not replacements, they are old bulbs.  One is extremely dim with a black end, the other dim one flickers, but doesn't have any black on the bulb from what I can tell. 
I have no idea how old any of these bulbs are except that they are not older than 1995, thats when this building was built.  I'm not sure how many of these lamps have been replaced in that time.  I do know that MH bulbs usually don't last that long, but I have to wonder if these two extremely dim ones might be originals??  I have never seen a MH dim that much before.  I would love to get my hands on these two bulbs!!

Are these ceramic metal halide lamps by any chance if so then the arc tube may be leaking and making the outer black 

And as for the one that's running dimly the other day I saw a quartz MH cycling warming up just enough so mercury was vaporised then it would turn off constantly doing that  I don't know what brand it was or what wattage it was

Are those sylvania protected lamps because there are some sylvania protected lamps in small high bay type fixtures at my local super market and a few of them have burned out and they are black just like you described.