BG - here is my opinion
Well on the question of will it block sound. I have found that when I put this in my vocal booth, it did help block more sound than was originally being blocked by the triple sheet rocking and blue underlayment that I had place on the walls.
Not much though.
The soundproofness of your closet is going to be determined by the way the walls were built and stuff, originally. The only thing you can do now is go back and try to do afterwards sound proofing.
ummm....minifusors and wedgies:
Minifusors is just a name for one of Auralex's diffusors, and Wedgies are just the name for there 1x1 Sq Wedge(Blade) foam. This pertains to the shape of the foam....some are blades or wedges, others are pyramid shaped, etc etc. Diffusors are basically objects place on walls in studios, to keep waves from standing(bouncing back and forth). They are used to break up the angle of the wall they are placed on, and create multiple angles to stop the sound from bouncing back and forth from wall to wall. They come in ALL shapes and sizes. There are theories on which shapes work better than others as well.
Have you actually recorded in the room? I would try that, if you have, and you found that the room wouldnt give you that upclose and personal vocal recording that you like, then I guess 99 bucks wouldnt be a bad purchase, cause you can always reuse the kit, as long as you dont glue it to hard to the wall.
I would think that it would at least take away the reverberation sound, if you could line the wedges up, around the wall, making a continous line (360 degrees). You have 1ft of up or down to play with, so you can position them on the wall for and average height and see if that works. If 24 will make it all the way around.
Do you have any pics of the closet? or a snippet of a recording you have done from the closet? If so let me know you could post them here or something, and see what kinda help we can suggest.
R.