I would dare say that Dark Blast is one of the best sets to go with FF. With FF you generaly run into two types of builds.
The most common is the holy trinity build which largely ignores its primary except for the three bubbles (dispersion, deflection and insulation).
The other is the spacial control build which uses the entire primary and specializes in controlling mob positioning (repel, force bubble, etc..).
Dark Blast helps both of these builds quite a bit. For the holy trinity folks, dark gives you the ability to spend half of your time playing the part of either psuedo-blaster or psuedo-controller. The damage output and mitigation of Dark Blast is nothing to sneeze at.
For spacial control, Dark Blast has some very useful tools.
1. Tenebrous Tenticles. TT is an imobilzation power, but it doesnt stop mobs from being knocked back or repeled by your other powers. This allows you to manuever mobs to your hearts content.
2. Torrent: This knockback cone gives you another power to constantly keep your opponents off of their feet, and gives you a little more flexibility for moving small groups of mobs.
3. Dark Pit. This isnt a spacial control power per say, but it does give you the ability to control the amount of agro you get from using all your other positioning powers. Because of the nature of the FF primary you will always have a much lower defense then your team, and pushing mobs around seems to generate quite a bit of unfriendly attention.
FF can be very similar to Storm Summoning in this regard.
But that's just my opinion
