This was a perfect plant. This started happening a week ago. All the leaves have gone from bright green to black. Some of the leaves are mushy.
I water this plant once a month.
If the leaves went mushy, then you have a one of two things that happened. One is over-watering. The other is a cold snap. If its a short cold snap the roots will still be alive, but the top will be mushy.
I am guessing its a case of over-watering or poor drainage. I believe you did not no over-water, but gave the plant poor drainage to start. When planting aloes you want at least 50% drainage matter, e.g.; gravel, perlite, pumice, lava rock, etc..., but you are much better off with 75-80% drainage to 25% brown matter (water holding). Without the right amount of drainage matter in any potted plant the brown matter can hold water for weeks without drying out.
It is also important to make sure you DO NOT put a layer of rock in the bottom of the pot. The same potting mix should be from the top to the bottom of the pot. This is help with drainage.
If you are unsure if your plant needs water use a bamboo skewer or chop stick. Insert it in the soil for 30 mins or longer. Then pull it out and check it. Look at it and touch it. You should be able to tell if there is still lots of moisture in the soil.
your plant can't get water from anywhere. give it water more regularly, and put wood chips under it. then it will send a long tap root down into the soil.
This is a very accurate description of what i think the problem is: https://www.ted.com/talks/allan_savory_how_to_green_the_world_s_deserts_and_reverse_climate_change
it also looks like people have been stepping all over it, and crushing the leaves.