I think it might be a bad idea for a DM to allow such a feat. Eldritch Blast and Invocations are rather different from such minor abilities as Smite Evil or the like. Smite Evil is only useable a few times per day and only adds 1 damage per level, to a melee attack only (generally going to be a normal melee attack). Eldritch Blast is 1d6/2 levels or so, ranged touch, and useable at will. While Invocations are likewise useable at will.......
A warlock 1/rogue X or whatever would gain a lot from such a feat, at practically no cost. Plus they could already get at-will invisibility through an Invocation, IIRC, by taking some levels of Warlock. Adding a full-power Eldritch Blast for ranged touch invisible sneak attacks (at almost full power from focusing on rogue levels) would probably be too much.