And I have no idea why it double posted. Sorry. I was wondering why it seemed like it was taking so long to load the page...
There are a number of mastermind primaries that mesh quite well with dark miasma. Play them all if you like to get a feel for them. In my opinion they are all very capable just depends on your temperament.
Robots/dark - very powerful. Positioning with the knockback, you eventually have a combo with incendiery missiles on tar patch that is quite lovely.
Necromancy/Dark - Your minions will all tend to move into melee. With the combines powers of your debuffs and heals around yourself, your mastermind will be safe enough and will want to heal the minions and stay right in battle with them. Sometimes you might have to nudge your second tier guys to more immediate melee, but I don't find the zombies all the difficult to manage, maybe just a touch more than robots.
Demons/Dark - This comination is newer, but it's a very powerful and effective synergy. Again, your mastermind will want to stay right in the thick of things keeping the demons healthy. In addition your ember demon will shield and heal YOU, too. Your demons are bigger than other mastermind pets, but don't take a back seat to anybody. Your guys are most effective in melee range. I don't monkey much at all manuvering demons, BUT I often split their stance, meaning I put the top 3 big boys on Attack/Aggressive and the little three first tier guys on defensive/follow. Obviously I don't do this in boss fights. I want that Demon Prince to fire off his first attack on the first target and that is usually his single target ice hold, and that suits me very fine.
Thugs/Dark is another powerful set. The only thing I micromanage is the bruiser. I have a Goto/Aggressive in situations where I want him to go and grab aggro and take the alpha strike. I don't do this every spawn. The twilight grasp coupled with the enforcers leadership and the pet aura IO sets make this set up extremely tough to kill. The added layers of tohit debuff and resistance debuff counter any concerns you may have with dealing a lot of lethal damage. Thugs just deal a lot of damage, period.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
For a "first primary" I would suggest a (mostly) ranged set.
Robots, Thugs, Mercenaries would all be excellent and would work fine with /Dark.
For Thugs, you get the Bruiser later at 26. Plenty of time to get used to pet controls before then. You'll have the arsonist to practice on for 8 levels.
Overall of the three ranged sets I'd go for Thugs. They're pretty nice all the way up and less clanky and noisy than the Bots (who also take a while to bloom).
Personally my first villain was a Necro/Dark which is a master of control and debuffs and not as hard to play as one might think. But then I had previous experience with an Illusion/Stormie.
Necro/Dark has the advantage of combining melee pets with a melee summoned Dark Servant and his wide-area heal, as well as stacked -ToHit from the Lich and yourself/Fluffy (stacked Fearsome Stares are lovely).
With the glitch that make minions run into melee, a ranged type isn't always the best since their powers are as the name inplies. I just made a necro/dark mm and im loving him, lvl 35. The tier 1 zombies run straight in to weaken the enemy while my tier 2 quickly fires a blast before swinging that bloody sword. They are mostly melee but do have some range as their tier 3 is just a dark/dark defender in a sense.
I'll put in another vote for necro/dark. The recent AI *ahem* changes were made to benefit demons, but they happen to also help your zombies and especially the (previously inept) grave knights. The Lich seems to behave about the same as before, which is not a bad thing.
Necro/dark isn't a steamroller of pwntastic AoE damage but it does feel very safe. Your pets will be throwing out some debuffs and heals that are similar to your own but I don't find it redundant, having "more of the same" allows me to tackle bigger crowds.
Incidentally dark miasma is so awesome you could pick attack squirrels as your primary and still make it work.
Hey!
I'm a new MM player as well...let me give you a pointer, regardless of which primary you run with.
Check out Sandolphan's MM keybinds here. It will make life easy.
Here's how it works...look at your Numpad. Buttons 1-3 will select that kind of henchman (1=tier 1, 2=tier 2, 3=tier 3) with 0 selecting all of them. Easy.
4,5 and 6 will choose aggressive, defensive or passive stances.
7 gives the 'attack my target' order; 8 is the 'follow' command and 9 is the 'goto' command.
After just a couple minutes of play this becomes incredibly intuitive. In literally 2 seconds you can issue different orders for each kind of pet. I also mapped the regular TAB function of cycling through enemies to the / key on the numpad and 'follow' to the asterisk; I have literally played through some mishes with one hand on the keypad and the other holding a drink. Not recommended for PVP. 
Anyway, it'll make you a master of masterminding.
So I have tried several Masterminds but haven't quite found my niche yet.
I've decided to play Dark seconday because I know and like dark, but I'm not sure what a good primary would be for someone who is kinda new to masterminds. I know Ninjas and Necromancy is probably not good for a relatively new mastermind player since they require very careful management, and bots probably isn't best with the knockback.
I was thinking demons but they are really big and it's hard to see what's going on when I am on teams with Demon Masterminds.
Thugs maybe? They have the brute though, and I'm not sure how difficult it is to control that guy. And I hear mercs are no good.
I just downed every mastermind set, didn't I?
See? I just don't know what's a good set for a relatively new player.
Suggestions? thoughs? ideas? Number of a good therapist?