It doesn't say that specifically, but if you go to CArc, at the beginning of the PrC chapter, it explains that +1 spellcasting PrCs increase Eldritch blast damage. It's an extrapolation of that clause, stating that Eldritch Blast damage is based off Warlock CL.
+1 spellcasting PrCs increase spellcasting as well. So an equivalent extrapolation would be that practiced spellcaster gives extra spell slots, spells known, etc for a sorcerer.
..in other words, I don't buy it at all.
Practiced Spellcaster gives Caster Level. Spell slots are calculated independent of caster level, EB is not.
No, EB is not calculated via caster level. Read the paragraph carefully. You kinda have to take the whole thing, as the invocation caster level and the eldritch blast caster level (since changed by errata)
and the eldritch blast damage were all changed.
Two examples:
A warlock taking levels in such a prestige class does not gain any of his class abilities, but he does gain an increased caster level when using his invocations and increased damage with his eldritch blast.
Note that these things are not the same as each other, and are explained separately.
(treat his combined caster level as his warlock class level
It's the effective
warlock class level that determines eldritch blast damage. Caster level goes up as well, but that's not so connected to the damage. Look at adding enlightened spirit levels to a warlock build. The caster level grows normally, but the eldritch blast damage follows the enlightened spirit advancement instead.