I am trying to make sort of a Dark toolbox deck and I am wondering what pokemon and trainers are good for such a deck. I also would appreciate any opinions on Dangerous Energy, on whether it is worth it or not.
Format? You'll get at least slightly different answers for Expanded versus Standard (or Legacy if you're a PTCGO player like me).
Dangerous Energy has a fun name but it isn't actually that dangerous due to the timing. If it placed a damage counter on an opponent's Active Pokémon-EX between turns* then you would be able to ensure at least one damage counter was placed and thus some level of return. Instead it triggers when your Darkness Type is attacked by a Pokémon-EX. So most decks have a couple options to deal with it directly or indirectly:
Discard it with Crushing Hammer, Enhanced Hammer, Xerosic or another card. While not a universal option in that not all decks already run such effects, some do and most have the option. In fact most decks would run these cards if they had space, and its usually a matter of how Special Energy removal waxes and wanes as the best tech tactic.
Attack with a non-Pokémon-EX. Some decks won't have to worry about triggering Dangerous Energy in the first place, and many (if not most) have a non-Pokémon-EX attacker to deal with other hassles; dealing with Dangerous Energy just gets added to that list of hassles.
Attack something else.Lysandreis a staple after all, and you don't have to KO each and every opposing Pokémon to win by Prizes.
On top of all that, are the extra damage counters from Dangerous Energy actually relevant to damage calculations? If it isn't changing the KO turn count, so that a 2HKO becomes an effective OHKO, or 3HKO becomes an effective 2HKO, then no it isn't.
*Not me trying to turn this into a Create-A-Card discussion; I just wanted an example to help illustrate the point
You can find several Darkness decks and Black Box variants in the deck section. You can also look into decks focused on Zoroark and decks focused on Darkrai and Garbodor with an without Giratina. There is also the option to run Darkrai with Hypno. For a more detailed discussion on Darkness Pokémon in standard and expanded check out google. In terms of cards from XY11: Steam Siege look into Ninja Boy amd Yveltal BREAK.