After taking another look at these invocations:
Castigating Blast
Greater; 6th; Eldritch Essence
Imbuing your eldritch blast with holy power, you instill overwhelming fear and remorse in your enemies.
Actually this looks more evil than celestial...

Circle of Peace
Lesser; 3rd
So long as you do not raise your hand against another, an aura of serenity and calm surrounds you.
This is more like it>>.

Discerning Sight
Lesser; 4th
The eyes of heaven are not easily fooled by trickery. By using this power, you can invoke the keen eye of an angel.
This would be quite useful

Eldritch Wrath
Dark; 9th; Blast Shape
When unleashed, the anger of the divine is terrifying to behold.
A very powerful blast shape, especially against multiple targets.

Eldritch Retribution
Greater; 5th; Blast Shape
Most often, holy forces do not strike the first blow. However, their righteous vengeance is swift.
Probably best for vindictive players.

Dark; 8th
The malign influence of demons and their servants works its way in deep. However, with divine power, they may be driven out.
Another dark invocation that is quite useful against summoned creatures, as well as mind-affecting attacks.

Guiding Spirits
Lesser; 4th
You have friends in high places. When you are uncertain, you may call on them to direct you towards the right path.
The warlock's equivalent to the augury spell. Best used sparingly.

Hands of the Healer
Greater; 5th
This invocation is equivalent to several cleric healing spells, including remove curse.The caster can also substitute a caster level check for a Heal check.
Quite a useful invocation, especially if the party has no cleric.

Radiant Blast
Least; 2nd; Eldritch Essence
Light is not innately holy, but it is a potent symbol, and one often used by beneficent powers.
This invocation is quite appropriate for celestial warlocks.

Transcendent Language
Lesser; 4th
Celestial forces are often called upon to be messengers and spokesmen of higher powers. Because of this, they are granted the ability to comprehend any language, and discern falsehood.
Being able to understand any language and having the ability to detect ghosts are both rather useful.

Valiant Martyr
Least; 2nd
Championing a cause is not an easy task- one must be willing to fight to the very end and against all odds.
Probably best for a wizard/ barbarian gestalt.

Vivacious Blast
Least; 1st; Eldritch Essence
Another useful invocation that not only damages undead, but partially heals living beings.