My Jet Contractor's saw does not have a flat on the blade shaft. To hold the blade while tightening or loosening I have the following shop made tool:
Take a 1 x 4 about 17 inches long. Place two 8 x 1" round head brass wood screws 3/4 " from one edge of the board, 4 inches from each end, leaving enough shank exposed to slip into a slot on the blade.
It is used by holding along the blade with the screw in a slot on the side of blade that would be forced downward by loosening the blade. When tightening slide the board so that the other screw holds the downward force on the blade.
Drill a hole in one end to hang it by and Bob's your uncle.