Njoy,
I think the garbage bags are ok for the leaf mold since making leaf mold (I'll actually have to try that method.) Making leaf mold isn't about a carbon to nitrogen ratio/hot composting. I would be concerned if you are trying to make "quick" or hot compost in a sealed garbage bag. Hot compost is an aerobic process and requires oxygen. A sealed garbage bag would run out of that before you got good compost.
I think I know where you are going though: the sealed bag keeps the moisture in, black would help keep the temps up, and a half filled bag wouldn't be all that hard to "toss" as Longy had suggested. In the end you'd have anaerobic decomposition, and I'd step back right after you opened the bag. If you waited long enough, it would turn into mush or soup depending on how much moisture was in the materials. It probably would not get hot enough to kill any
seeds that were in the bag too.
Over the winter, I keep my kitchen scraps in recycled 40 lb cat litter pales. By
spring, I have mush and soup. It's great for starting a new pile, but it smells bad, and I've gotten tomato and melon sprouts if I don't hot compost the material.