Well, looking in FC2, we find ... not much clarity. On the plus side, in the sample statblock, hellfire blast is not included in the list of incantations known. So it's probably not an incantation per se. On the other hand, hellfire blast is listed seperately from eldritch blast in several places. This could just be because it does more damage and costs Con, but it could be interpreted as implying that hellfire blast is considered a seperate ability and thus the various blast shape, etc. invocations, which specifically apply to eldritch blast, could not be applied to hellfire blast. I wouldn't go that far myself. The rest of the entry is identical, AFAICT, to the excerpt cited earlier.
So, a strong implication that hellfire blast is a modification of eldritch blast, not an incantation. But nothing definite. Bleah.