My recollection of U-571 is hazy (thank goodness), but if the two submarines were at periscope depth then I dont see why a blind shot should not have been possible. There is a story of a submerged RN submarine sinking a submerged U-Boat, in fact I think that the RN submarine may even have repeated the feat, but it would not have been possible if the submarines were deeper than periscope depth.

I dont want the above comment to be understood as supporting the historical veracity of U-571 in any way.

Andrew