The black leaves would mean they did rot. So too much water in the soil or staying wet too long.
When they grow tall - not enough light.
If I remember right you put them all together on one big pot. It might have been too big for them and the soil didn't dry. If you can take the remaining ones and plant them in separate small post (not bigger in diameter than the plant itself). They do not need much space and feel good when the roots are tight. Use more sandy soil or mix the universal plant soil with some sand/grid/perlite/rocks. Do not water until they are completely dry. It is better to keep them dry a bit longer than wet.
I may have to try again when I have the correct kind of pots. I put mine out in the full sun and now the squirrels have been into them. I'll just enjoy yours for now.
Oh boy :(
You can try to pot them in some small plastic coffee cups. Like the ones for espresso. I normally do that with smaller succulents :)