Btooom was suppose to get a second show, but wasn't popular in Japan. Btooom has a dark feel mainly because of the characters' back stories (the mention of rape comes up a lot). It's good in a way, not too gorey almost like Death Note/Code Geass with the anticipation planning and a brilliant amount of suspense.
Unfortunately I've never seen nor heard of Yuki Yuna is a Hero, sorry.