If you created your Xcode project based on one of the standard templates, the Cocoa headers (including NSException.h which contains the NSAssert macros) will get preprocessed before any other files in the project. A #define NS_BLOCK_ASSERTIONS in any of the project’s header or implementation files therefore has no effect on the NSAssert macros.
Try putting NS_BLOCK_ASSERTIONS into the preprocessor macros of your target or even project (for the release configuration only):
Or put #define NS_BLOCK_ASSERTIONS into the prefix (.pch) header before the #import <Cocoa/Cocoa.h> or #import <Foundation/Foundation.h> lines.