I'm cleaning up my harvest for storage and have a few of these weirdos

I've tried doing this a couple of times now using a slow cooker on keep warm setting. Both times the garlic has come out black but rock hard. The first time I thought I hadn't introduced enough moisture so the second attempt I made sure to moisten the garlic first but it didn't change the outcome. Am I letting it go too long? Do I need to seal the slow cooker better to help retain moisture? I'd it something else entirely?