Can Ground Blinds really help? When used correctly... Yes!
Are they necessary... No!
I can understand the need for ground blinds for hunting White-tailed deer or other game that have sharp eyesight. Deer can detect the slightest movement or discern thing that are out of place from hundreds of meters away; if they do they also have the uncanny ability to determine if there is any threat involved. Of course if danger of any kind is determined they will leave... in a hurry!
Moose on the other hand do not have the keen eyesight like other deer, but they do have other sensory faculties that can give you away.
A natural blind, with a few extra branches propped along the sides for concealment.
In my opinion I do not believe the use of a blind for hunting moose is required. I have been as close as 6 meters to a moose without the use of a blind. Using the natural terrain or trees to breakup your silhouette is as effective as setting up a blind.
For instance, during a 2009 moose hunt in British Columbia my son and I concealed ourselves (if you want to call it that) by sitting in front of small lodgepole pine trees. We were both dressed completely in camouflage clothing but we did not go to the extent of covering our faces. From where we sat we had a reasonable view of a treed meadow that was backed by an grown in logged area.
We sat and called using a birch bark moose call. After some coaxing we managed to call a nice bull moose to within about 20 paces before we harvested him. I cannot say if he would have come any closer (I did not want that!). He stood staring directly at me. Could he see me? I am not sure! He would have been looking for a cow moose, so as long as I did not move I do not believe he knew I was there.
Never having used a purchased blind for hunting moose I am a bit biased. Maybe they would help, personally though I want to be a little more mobile that what a blind can offer.
I did however buy one of those camouflaged burlap tarps, thinking that I would or could stretch it between some trees and use it to conceal me... it is still in the wrapper! It is just one of those things I never get around to using!
Personally, I am not aware of anyone uses a purchased blind for hunting moose, there are a few of the guys I know who use tree stands, some portable and some permanent.