There is an excellent thread over at ProSoundWeb on the mysteries of the Blumlein array.
recforums.prosoundweb.com/index.php/t/853/0

and it includes a great link to a stereo image visualisation tool...
http://www.hauptmikrofon.de/ima2-folder/ImageAssistant2.html

Definitely worth a read. Excellent clear writing by DAvid Satz as usual..