The warlock's ability to regulate (to some degree) the effects of her eldritch blast (like damage) by lowering the blast's effective caster level has no impact on her ability to pick or employ metamagic-like feats related to the eldritch blast.
The eldritch blast's spell level
The warlock's effective caster level of her eldritch blast isn't the same as the eldritch blast's effective spell level. (This overuse of the word level dates to the game's inception, even inspiring a favorite Web comic strip.) The spell level of eldritch blast was changed by the Complete Arcane errata:
Page 7: Eldritch Blast
Second paragraph of the Eldritch Blast ability description: Change “An eldritch blast is the equivalent of a spell whose level is equal to one-half the warlock’s class level (round down), with a minimum spell level of 1st and a maximum of 9th when the warlock reaches 18th level or higher” to “An eldritch blast is the equivalent of a 1st-level spell. If you apply a blast shape or eldritch essence invocation to your eldritch blast (see page 130), your eldritch blast uses the level equivalent of the shape or essence.”
Any other references to eldritch blast being something other than the equivalent of a 1st-level spell should be disregarded. Any other references claiming that eldritch blast is not an invocation should be disregarded. A warlock can use eldritch blast at will. (1)
This means for most purposes except as pointed out by the errata the eldritch blast is effectively a 1st-level spell. Thus, for example, a warlock must possess at least a caster level of 6 (therefore usually being a level 6 warlock) to take the feats Empower Spell-like Ability (eldritch blast) (Monster Manual 303) and Maximize Spell-like Ability (eldritch blast) (Complete Arcane 81), and a warlock must possess at least a caster level of 10 (therefore usually being a level 10 warlock) to take the feat Quicken Spell-like Ability (eldritch blast) (Monster Manual 304).
The eldritch blast's caster level
Because, except for a few situations (like, for example, its lack of components), "a spell-like ability functions just like a spell" (PH 180), this means an eldritch blast can be employed "at a lower caster level than normal, but the caster level you choose must be high enough for you to cast the spell [i.e. employ the blast]…, and all level-dependent features must be based on the same caster level" (PH 171). Thus, a level 7 warlock could opt to employ the eldritch blast as if her effective caster level were, instead, 6, 5, 4, 3, 2, or 1.
For example, to bring this all together, if she wanted to for some reason, a level 7 warlock that possesses the feat Empower Spell-like Ability (eldritch blast) can use the feat yet employ the blast as though her effective caster level were as low as 1.
It’s unclear, unfortunately, whether the “selection” is a thing done once, when you take Quicken Spell-like Ability, or if you must constantly re-select the same choice every time you want to use it—that is, whether or not you have to re-check the spell-like ability’s spell level each time.
If you don’t, then as long as you had CL 10th when you selected eldritch blast (which is necessary to meet the feat requirements anyway), you could select eldritch blast as the spell-like ability in question, and then no matter what changes happen later, you could use quicken eldritch blast.
On the other hand, if the selection is something you have to do again each time you use Quicken Spell-like Ability, then you have an issue. A brimstone blast may still “be” eldritch blast for the purposes of being the spell-like ability you chose originally, but brimstone blast does change the spell level of that spell-like ability to 3rd. Until CL 14th, Quicken Spell-like Ability would not allow you to select a 3rd-level spell-like ability. Therefore, you would have to wait until CL 14th to use it on a brimstone blast.
I feel that the rules text favors the former interpretation, of selecting just once when the feat is taken, but the rules text was also definitely not written with consideration for spell-like abilities that change spell level. That’s not something that spell-like abilities do—except for eldritch blast.
Best Answer
Both Mind blast is both a spell-like ability and a special attack.
The DnD3.5 has a category of character/monster properties called Special Abilities. They include extraordinary, spell-like, or supernatural abilities. These abilities appear in the monster entries either in Special Qualities or in Special Attack entry. It depends on the Special Ability function, whether it is used to attack or to defend. So, a character/monster property can be both a Spell-like Ability and a Special Attack at the same time.
To find out how the ability interacts with magic (including metamagic feats) you need to know whether it is extraordinary, spell-like, or supernatural. Whether it is a Special Attack or a Special Quality does not matter.
In your case mind blast is a spell-like ability and its level is not specified, so by RAW:
(emphasis mine) link
I.e. all prerequisites for Quicken Spell-Like Ability monster feat are met
link
But the description also restricts the level of the spell the spell-like ability imitates. The table in the link above lists 16th level (i.e. 16HD for mind flayer mind blast) for 4th level spell (according to mind blast description it imitates 4th level spell).
You can NOT use Quicken Spell-Like Ability monster feat with 12HD mind flayer Mind Blast. You need at least 16HD.
Whether the property is a Special Attack or a Special Quality matters when applying templates, for example.