Some more information about error reporting with Blesta. This clears up confusion for me.
Errors are reported to screen, even with display_errors set to off, as errors are trapped by file app/views/errors/error.pdt
The boolean internal variable System.Debug decides whether error.pdt includes a stack trace or not
To make sure no errors are visible error_reporting must be set to 0, such as by using Configure::errorReporting(0);
Errrors are also logged to various files in logs_blesta directory