Here's a shot that shows the whole plant...

I'm not sure if the black spot has anything to do with the crack on the leaf. It might be a sunburn but depends how long it has been black, usually sunburns will callus over time and become white/light brown-ish. But if that is soft and you said it's expanding, then it might be rot problem.. I would suggest to check the roots and see if there's rotting there. Also, it looks like this is planted in the ground, and it looks a bit over"potted", but I can't really tell from just this picture

Thanks for the reply. The black spot has been there for a week or ten days. It has been expanding. The whole-leaf discoloration is new in the last couple of days. The plant is in the ground: I don't know what over-potted means (as I said, I'm new at this), so please fill me it.

If it is rot, would it be appropriate for me to remove the leaf from the plant?

Overpotting basically means you're burrying too much of the plant in the soil, leaving too many of the leaves touching the ground. But I think yours fine actually.

How much sun does your echeveria get? And how often do you water it? Weather conditions in your area?

I'm not experienced with dealing with rot either so I would wait for others to chime in, but usually you need to take the whole plant out and cut off the rotted parts of the roots.
I also just noticed that the leaf is turning yellow, so perhaps it's just that particular leaf withering and dying, which is perfectly normal, so yeah I would say you can just take off that leaf for now.

It does look to be rotting a bit, but I would start by just plucking off that leaf. It could be that some water pooled on it at one point and that is why it's rotting in that spot; if the blackness isn't emanating from the stem of the plant, I don't think it is a big issue. Just keep and eye on it and make sure this doesn't continue to happen. If that leaf was cracked, most likely it was dunzo anyway.

It isn't too important at this point what the spot is or is not; none of he other leaves seem to be affected. The leave is turning yellow naturally because it is dying. I doubt the spot has anything to do with it. Echeveria leaves begin to turn yellow and die from the bottom up. This leave may be turning yellow bit sooner than it otherwise would have due to being transplanted to a new home; again normal situation... it just happens. As long as the leaves are firm, there is no need to dig it up. For now, just remove the leaf. I routinely remove any Echeveria leaf that begins to yellow. You will want to grow this plant with lots of light, but maybe not direct sun mid-day...might be too much.

This post was edited by bikerdoc5968 on Sat, Jul 5, 14 at 8:05

Thank you all so much for the help. You gave me the courage to go out and remove the leaf, and I found that the plant had done so itself--the leaf was completely detached, sitting in the dirt next to the plant. I'm optimistic about its future: it is in a narrow shady area between the side of my house and a fence, and below a neighbor's bottle brush tree, so it gets a fairly brief period of exposure to direct sunlight and a lot of shaded reflected light. I'm in Los Angeles, the eastern part toward Pasadena, so I assume the climate won't present any problem so long as there's sufficient water. If I've got any of that wrong, please let me know. Thanks again.

Most Echeveria enjoy basking in the sun. Mine summer in full sun until the fall temps force them back into the greenhouse. For you, living in southern CA, too much mid-day sun might bleach out the color as well as run the risk of having a crispy critter. So you want to pick a location where they can get several hours in the morning and then late afternoon sun. Otherwise, they may not have good color or tight growth. You'll have to experiment for your specific location what will work best. It's a nice Echeveria and I think you want to keep it that way.