I had issues with that too, but in the end I concluded it was OK. The sleeves are loose, drape like. No different in principle than dealing with the loose edges of a conventional dark except that the Blackjacket does not dangle open or flap in the wind as much. It has an adjustable shock cord to secure around the camera. That is better than velcro which can wear out and attaches annoyingly to camera cases etc. It is better than elastic because it does not torque the camera during on/off and sometimes mess up focus. Also, the cinch cord makes it easier to use with multiple cameras or formats than elastic types.
Good points that I'll consider. Thanks for taking the time.