Yeah, the visual difference is that the end (where the bullet would normally be) is crimped. Wikipedia has pictures of blanks:
http://upload.wikimedia.org/wikipedi.../7.9Blanks.jpg
The crimp on a blank means that many guns, particularly short-recoil semi-auto pistols, have to be modified to eject them properly. A lot of pistols need to have their ejection ports widened and/or lock-up removed in order to cycle blanks.