Blanks are live ammo, and should be treated with the same respect due live ammo. When the trigger is pulled, that large volume of rapidly expanding searing hot powder gas has to go somewhere, and at close range, it can kill -- and has, in the past. If a demonstration of a blank cartridge's power is deemed necessary, an apple or orange can be sacrificed for the cause; make sure you are in a location safe for firearms discharge, press the muzzle against the side of the fruit (while still pointing in a safe direction), and pull the trigger.
Banks are manufactured in different ways, and some can be more dangerous than others. I can remember seeing imported (from where, I forget?) blanks that had the case mouth plugged with a small piece of wood, instead of the case itself being crimped closed like a shotgun shell. When fired in the original military weapon they were designed for, a special blank adapter would shred the wood plug as it exited the barrel, causing the splintered pieces to lose velocity and become less dangerous more quickly. If fired in a weapon without this special adapter, the plug exits the barrel unshredded, at a very high rate of speed, just like (you guessed it) a regular bullet! Less accurate, but still lethal at close to moderate distances.
Some blanks are loaded with black powder which can leave a corrosive residue behind when fired, so promptly cleaning any gun they are used in is essential.
Good enough for a "Blank Safety 101" start?