while looking on in blank amazement; while gazing at it in shock
Describes a situation where one watches something unfold in stunned disbelief, unable to react. Often used when events happen too quickly or unexpectedly to intervene.
See also: あれよあれよ
While we looked on in blank amazement, he fell off the stage.
As we gazed in shock, the fire spread throughout the house.
The expression combines the interjection あれよあれよ (expressing surprise) with 見る間に (while watching). The exact origin is uncertain, but it likely emerged from colloquial storytelling.