For what it is worth, the Compendium states:
Pact Boon: Each pact includes a pact boon. The pact boon is a granted power you can use to further hex your enemies."
More to the point, the PHB also contains this wording (p. 130), and you can't get much more primary a source than that.
As secondary, anecdotal support, I'll mention the D&D comic book (I know, far from authoritative) which in the back of issue 4 has the character sheet for a Star Pact Warlock. It lists "Fate of the Void" as an at-will power, so there is at least some indication that even the original PHB pact boons are intended to be powers.
(As I do not have a DDI subscription, I will ask: how are pact boons formatted & presented when printed? Are they given the format of a power?)
So, even though the pact boon is not presented in the format that other attack and utility powers are, I could see a case being made that it is indeed a power.
However, in regards to being able to use Astral Fire to boost the damage from the pact boon, there is another consideration. Astral Fire and similar feats specifically apply to powers with certain keywords. I do not have the FRPG, so I will ask: does Dark Spiral Aura, not being presented in the standard format for a power, contain a list of keywords? If not, then it doesn't have the 'radiant' keyword, and cannot benefit from Astral Fire.
Alternatively, is there a rule that makes it explicit that if a power deals 'x' type of damage that it
must have 'x' keyword? The best I can find is that if it has 'x' keyword it does 'x' type of damage, which is not quite the same, and also rules that pertain to adding or removing a damage type, which again does not quite address the question at hand.
-Dan'L