IMO this is probably the most misunderstood aspect of playing Blackheart. I've been told that I have an almost intuitive understanding of how the demons work. Since I don't believe in intuition, I'm going to share what I know in hopes that I can teach someone:
FIERCE demons, if they connect, latch onto the opponent and drain their life. If you connect with fierce demons, it might be good to not throw any demons or do any special moves for awhile, especially if they're almost dead. Otherwise, the demons will disappear and will gnaw away no more life.
ROUNDHOUSE demons hold the opponent, but this is almost useless since the opponent can shake out easily. Also, Sentinel can just run through roundhouse demons without flinching because of the damn super armor (Hulk and Juggernaut can too, but they suck so who cares). However, if your opponent doesn't know about this, you can sometimes connect a super move afterward. However, if roundhouse demons connect mid-air, they'll bounce upward which gives you an opportunity to do AAA into inferno xx super, or the infinite if you're stupid.
STANDING FIERCE - not very useful, although because of its initial chest bump it can be semi-useful against constantly overheading Magnetos.
STANDING ROUNDHOUSE - you need to be careful with this move because there's a space between the initial kick and the demons where the opponent can call an assist. You can cancel the kick into Judgement Day and it will usually combo.
CROUCHING FIERCE - combined with Sentinel drones, c.fierce is a good way to lock the bottom screen down if someone's trying to rush you. Otherwise, no.
CROUCHING ROUNDHOUSE - generally useless. I've seen Viscant use this move, but I don't know why. There's no initial hit and the demons don't go out far at all. Don't use.
(SUPER)JUMPING FIERCE - THE MOST USEFUL ATTACK BLACKHEART HAS. Why people continue to overuse sj.rh is beyond me. Jumping fierce demons go out for a certain distance and then return to Blackheart. Not where he was when he threw them, where he is. This is incredibly useful because you can change the trajectory of the demons by airdashing in whatever direction, or jumping, landing and then superjumping. Sky's the limit.
(SUPER)JUMPING ROUNDHOUSE - Useful to cover assists and that's it. You can runaway with it from time to time, but only if the opponent is otherwise occupied on the ground (ie. there's an assist in his way).
Hope this helps upcoming Blackheart players.