yeh that would explain the darkness of the leaves, it doesn't show it in the photo from the cflzhow old are these? that might be a lot of early nutrients in the manure mixes for them? they look super young. you need to move them into separate pots like the other posters said.
This is the pics they were fine yesterday but now are drooping and some have dark brown dots and are shrivelling
have a look..
Thanks for the help!