Problems: This is nice and flavorful, but there's really no particular reason not to play a warlock this way.
And Eldritch blast is slower than sneak attack progression for a reason, i.e. It's a 60 foot touch attack. You don't have to feint, or flank, or hide. Just point your finger. Optimally, from 150 feet away. As an alternate class, this is pretty good. Except for Spell Resistance. That's handled silly-like. SR of 5+character level= 1/4 chance, on average, of avoiding spells. SR 10+Char level is half, and so on. SR 15+half character level is a little silly. By level 20, your effective SR(SR minus CL for that ECL) is 5. At level 15, your effect SR is 7.5. Change it to 5,10,15 SR plus your Fey warlock level.
And, didja know that 4th ed is doing something like this to the warlock?