By default, eldritch blast has no specific type and is limited to a range of 30', with no saving throw (but spell resistance applies), and has an effective caster level (ECL) of 1. Eldritch essence & blast shape invocations will modify the effects of the blast. This usually increases the ECL. If it has an additional effect that applies a save, your Charisma modifier will be applied to the DC.
A unmodified eldritch blast by itself ignores SR checks and spell mantles once you gain 10d6 eldritch blast damage. This seems to be because the game recognizes the spell level of the Eldritch Blast by its damage die. Once the damage die exceeds 9 the spell resistance code breaks, as it wasn't designed with spells higher than level 9 in mind.