This handbook suggests that
Flurry of Blows is an attack when recommending the otherwise extremely underwhelming malec-keth janissary:
Malec-Keth Janissary (Swordmage-MoP)- Add extra damage to all attacks. As this can be thunder damage, it can be combined with Resounding Thunder. I also believe the extra damage is applied to FoB, as FoB is also an attack. The rest of the path is pretty worthless, but you don't need anything else.
Primordial Blade (16th level): Years of tireless study and exposure to the forces within the Elemental Chaos have honed your ability to control the elements. At the end of each short rest or extended rest, select one of the following damage types: cold, fire, thunder, or lightning. Your attacks deal 1d4 extra damage of the selected damage type until the end of your next rest.
Centered Flurry of Blows Your fists become a blur as you follow up your initial attack with another, shifting your foes’ positions to your advantage. At-Will Psionic No Action Melee 1 Trigger: You hit with an attack during your turn Target: One creature Level 11: One or two creatures Level 21: Each enemy adjacent to you Effect: The target takes damage equal to 2 + your Wisdom modifier, and you slide it 1 square to a square adjacent to you, or 1 square in any direction if the target wasn’t targeted by the triggering attack. Special: You can use this power only once per round.
Flurry of Blows is listed as a "monk feature."
Flurry of Blows count as an attack for purposes of