A Soul Knife / Warlock sounds interesting, but its medium BAB, low fort save, and 4 skill points may make a slightly sub-optimal build compared to others. Not weak, mind you, just perhaps less than optimized.
I'd play one anyway, however, simply for the flavor. I'd probably make Int the #2 ability, to capitalize on the many class skills available. Dex would be highest, however, for survival reasons. Str would be last or next to it, probably, but otherwise I'm not certain of the order of arrangement of the ability scores. It would depend on what I rolled. And if the DM were willing to create a feat allowing the Eldritch Blast to carry Psychic damage, all the better (although I admit it is unlikely). Fun to consider, though. Even without that this combination would be very interesting.