Given that "eldritch" is frequently used with warlocks (i.e. "eldritch blast"), it wouldn't surprise me, in WotC products, to see "eldritch" come to refer primarily to warlock magics, as another option from "arcane" and "divine." I don't think this will be a formal useage--as in, there won't be rules for arcane vs. divine vs. eldritch--just a common one that writers pick up and take advantage of.
The Warlock Class is really what presented this question in the first place. I like the Warlock class, but in some respects it seems overpowered and others severly lacking. I guess that means it is balanced overall.
If you read the class description in Complete Arcane, it sort of hints that the "eldritch blast" is some sort of baleful magic (which I interpret to be some sort of dark magic). This just creates even more questions.