The "debris" rings would be gases and plasma circling the around the blackhole's event horizon in real life. The radiation would be caused by the extreme stress and friction of said gases and plasma orbiting the blackhole at high velocity. Again, that's real life theory.
As for gameplay mechanics: just make it a hazard in space. If you get too close to one, you start taking damage to your ship/fleet. Otherwise, you can treat them like a planet if you keep your proper distance. Of course, even if you do land dead smack on top of one, you'd still be able to escape. I wouldn't want it so that if you fell into an in-game blackhole that you couldn't get out.
If you want spome other effects, we can take the radiation aspect (assuming all blackhole in game are the same) as make it so the radiation requires you to have your shields up to prevent damage of some sort (I'd advocate killing crew slowly.) So if I had to sum up a balckhole mechanic for the game:
- Outer radius - Standard gravity field. Just like flying around planets.
- Middle radius - Radiation ring. Shields required for protection. Does shield damage, so you can only stay here for so long before taking damage. Can destroy you if you let it, but you can maybe still salvage the ship afterwards.
- Inner radius - Event Horizon. Continuous damage taken, regardless of shield status. Will destroy you (and prevent possible salvage) if stay in it too long.
That's the basic gameplay mechanic I can think of for blackholes. Not very realistic in the absolute sense (you'd be utterly screwed immediately if you cross an event horizon), but this is a game, so liberties can be taken for the sake of gameplay mechanics
