Completely stopping all vision and projectiles essentially cuts off all of Order and Chaos' options for DPS. It's invincibility, essentially.
I actually think it wouldn't be overpowered, because
1) It's not cheap: it costs 550 gold to make, which is almost 3 times as much as a miner wall. It's also difficult and risky to run an air and an earth up to the front line to combine them.
2) It's immovable: while it may prevent an advance, the enemy can simply fall a little ways back and the ele player can't do anything about it.
3) It's temporary: it's the only thing in the game that disappears after a certain amount of time. That means that if the opponent just waits them out, the blind gate will be gone, and nothing will have come of that 550 gold.
So yes, the blind gate grants invincibility. But the way I see it, it grants invincibility at a heft cost, it grants invincibility in a small area of the map, and it grants invincibility for a limited amount of time. Is that temporary invincibility better than getting an air and a miner, both of which are investments that are useful throughout the game? I suppose it depends on the situation. But a blind gate that blocks projectiles seems to me like a very powerful but situational tool, factors which I think balance it out.