Master Spellthief + Warlock + Unseen Seer = Eldritch Blast Cheese?

[Tonal Architect]

I'm going through some late night optimization of a character first referenced here -> http://www.giantitp.com/forums/showthread.php?t=216002

Stumbled across a post by Person Man, in which he claims that Hide in Plain Sight is achievable by taking a level of Warlock or Dragonfire Adept, choosing Darkness as an invocation, and then selecting the feat Blend into Shadows.

I never went through the Warlock's description until them, so I gave it a quick read to see how well it could synergize with other options.. Now, here's what caught my eye:

Complete Arcane, p18, on Warlocks and Multiclassing:

(treat his combined caster level as his warlock class level when looking at Table 1-1: The Warlock, to determine eldritch blast damage)

So, presuming a standard Unseen Seer build...

1 Spellthief
5 Wizard
10 USS

Throwing a Warlock level from somewhere in there, along with Master Spellthief, can I assume that, taking this build to the 20th level will grant the player not only full CL, nearly full spellcasting, AND infinite Eldritch Blasts along with a form of HipS?

Hm. In case one can Sneak Attack with Eldritch Blasts, I think this build shows a lot of promise.

[Elric VIII]

Throwing a Warlock level from somewhere in there, along with Master Spellthief, can I assume that, taking this build to the 20th level will grant the player not only full CL, nearly full spellcasting, AND infinite Eldritch Blasts along with a form of HipS?

This seems to be correct. EB follows the rules for weaponlike spells. Since it requires an attack roll you can apply SA and it crits on a 20 for x2.

However, there is some debate as to the interaction of Divination Spell Power and Master Spellthief. Namely, US is not technically an "arcane spellcasting class." If you get yoru DM to let that slide, there is the issue of stacking; does MS set your CL at a certain number, to be altered by Divination Spell Power, or does it simply eliminate the -CL, while allowing the +CL for divination?

EDIT: I just realized that you were asking if the "+1 level..." class feature adds to your Wizard casting and your Warlock Invocations/EB. You have to choose on or the other (the same as having multiple spellcasting classes). This can also be infered from the fact that the Eldritch Theurge has an advancement line for both classes, rather than just "+1 level of arcane spellcasting," to advace Warlock and Arcane Class X.

[kestrel404]

Your spellthief and arcane spellcaster levels also stack
when determining your caster level for all arcane spells.

Eldritch Blast is a Spell-like ability, not an arcane spell - you wouldn't get the bonus to eldritch blast damage, because this is not a straight up 'add X to caster level' but instead it's a 'stack X and Y for caster level to ARCANE SPELLS'.

It's actually a very annoying and limiting wording.

Also, unless you add a level of Sublime Chord to the mix, you've still only got 1 spellthief level, so even if your GM ruled that you get to stack Spellthief and Warlock (a reasonable ruling from a RAI perspective), you'd only get a +1 bump, not a +Wizard Casting bump.

[Tonal Architect]

Well, no cheese to be had here, then.

[Treblain]

Warlock 3/Rogue 1/Beguiler 1 can qualify for Unseen Seer, though. Alternately, if you don't want to bother with the Darkness trick, Warlock 3/Scout 1/Beguiler 1/Unseen Seer 10 gets you 6d6 EB and 5d6 skirmish at level 15. Fill out the build with 3 levels of Hellfire Warlock, one of Binder, and one more of Warlock, and you deal 20d6 with a single eldritch blast (counting improved skirmish), and get invocations as a 17th level warlock. And I think skirmish damage works with Eldritch Chain, so you can hit multiple targets!