The default for any invoker class is to be unable to choose any invocation except those explicitly assigned to it. Complete Arcane, Complete Mage, and Dragon Magic all explicitly assign various invocations to the warlock class. Complete Mage does not, however, include greatreach blast or spellblast in the list of invocations it assigns to the warlock, nor does it assign those invocations to the warlock separately when they are described within prestige class abilities. As such, those invocations are not available to a warlock unless or until he gains the class feature that offers them.

To see this, let’s start with the very definition of warlock invocations:

Invocations: [...] A list of available invocations can be found following this class description, and a complete description of each invocation can be found in Chapter 4 of this book.

^{(Complete Arcane pg. 7)}

The warlock can literally learn only those invocations that are listed in its description there. At no point in the entire column-and-change description of the invocations ability does the warlock gain any ability to gain any other invocation; the class feature only enables the selection of those invocations listed within the class description itself.

That doesn’t mean that these are the only invocations a warlock can ever take, however! D&D 3.5e is an exception-based ruleset, which means future material can, and does, make exceptions to earlier rules. That isn’t the point. The point is that the default state for any given invocation is for it to be not an option for a warlock. Something has to make it an option for a warlock in order for it to become one.

And the books consistently do exactly that when introducing new invocations. For example,

Also included are over twenty new invocations for the warlock class found in the Complete Arcane supplement. These invocations are governed by the rules in that book.

^{(Complete Mage pg. 89)}

That quote is from the beginning of Chapter 4, Spells and Invocations, of Complete Mage. It specifies that these new invocations are for the warlock class. That provides an exception to the rule on page 7 of Complete Arcane that says a warlock’s options are only those invocations included right there on pages 8 through 10 of Complete Arcane.

Not listed in Chapter 4, however, are spellblast or greatreach blast. Those invocations are described only in the descriptions of the relevant prestige classes—and those descriptions only give them to those prestige classes, rather than make them available to just any warlock. (In fact, the word “warlock” does not appear in the description of either class feature.)

For another example, consider Dragon Magic, which presents the dragonfire adept class, another invoker. These quotes are the references to its invocations:

Invocations: [...] See Draconic Invocations, below, for a list of available invocations.

^{(Dragon Magic pg. 25)}

Draconic Invocations

Each draconic invocation falls into one of four grades: least, lesser, greater, or dark. These invocations are described briefly below. See Chapter 3: Draconic Magic for full descriptions.

^{(Dragon Magic pg. 26)}

The invocations available to dragonfire adepts (see page 24) are described in this section. The format for invocation descriptions is given below, followed by the lists of invocations available to dragonfire adepts.

^{(Dragon Magic pg. 78)}

Note that none of these make any mention of the warlock class, and yet they are presenting “invocations,” and specifically, invocations graded into four tiers of least, lesser, greater, and dark. The invocation format does not include an associated class, as the spell format does, either, and the dragonfire adept reprints several invocations that Complete Arcane and Complete Mage had made available to the warlock, under this heading that says they’re available to dragonfire adepts as well.

And as perhaps the final nail in the coffin, Dragon Magic also presents several new warlock invocations. There are in an entirely different section, which reads

Because the powers of warlocks (see Complete Arcane) are tied to their dark nature, they're usually incompatible with draconic magic. However, warlocks living in a world where dragons are more active learn to tune their powers toward the destruction of such foes. These new invocations are intended for use by warlocks who expect to face dragons (or similarly formidable spellcasting opponents) in combat.

^{(Dragon Magic pg. 81)}

Note again the complete lack of anything here saying that dragonfire adepts cannot take these new warlock invocations; per Dragon Magic pg. 25, there is no reason to do so because these invocations do not appear in the list that begins on page 25, so they are unavailable to dragonfire adepts by default.

We have an existing Q&A about dragonfire adepts and warlocks and whether they can take each other’s invocations—and it comes to the same conclusion as this answer, that invocations are class-specific and can only be taken by a dragonfire adept or warlock if they are printed in a section assigning the invocations there to that class.

So yes, there is something preventing regular warlocks from taking those invocations: those invocations are simply never made available to them unless or until they gain the class feature from the relevant prestige class that makes them available.

The way it is written in UA, NO.

Note the use of is, instead of something like gains a +2 bonus

Any weapon you create using your Pact of the Blade feature is a +2 weapon. This invocation doesn’t affect a magic weapon you transformed into your pact weapon.

This is similar to the writing in Armors, in which your AC is something, and stacking armors (somehow) wouldn't stack bonuses.

The way it is written in XGtE, probably NO.

For the XGtE,

In addition, the weapon gains a +1 bonus to its attack and damage rolls

which would stack with another effect that said something on the lines of

In addition, the weapon gains a +2 bonus to its attack and damage rolls

except that it is explicitly states

unless it is a magic weapon that already has a bonus to those rolls.

and Pact Blade counts as a magical weapon (at least for resistances against non-magic weapons), as stated

You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it. You are proficient with it while you wield it. This weapon counts as magical for the purpose of overcoming Resistance and immunity to nonmagical attacks and damage.

While not explicitly stated that it counts as a magical weapon for the purpose of the bonus, it seems it is intended to not stack.

_{Note that the UA usually is not balanced (the point of it is to playtest and balance) and you should take care with using this content. From my experience, the writting was also not refined by then, as can be seen in the difference between the actual release and the UA version. You could suppose the intended purpose of what was written on UA is what is written on the official release, but that would not be RAW.}