Q156 Are there any prestige classes that by RAW increase Warlock's eldritch blast/invocations and increases sneak attack?
DM won't allow arcane trickster because it specifically calls for caster level arcane spell level :-(
Edit: got my facts backwards, but the question is still valid
Possibly Unseen Seer. It either requires arcane caster level 1, or ability to cast 1st level arcane spells; I don't recall which. Worst case, you dip one level of Wiz or Sorcerer, then use Unseen Seer to advance Warlock.
Another possibility, depending on what you want out of this character, would be to base it on Sorcerer, Wizard, or Warmage, and take Acidic Splatter for unlimited magical blasting. The damage would effectively advance on pace with a Warlock (with a Wiz base), though the range would suck for the first several levels.