They weren’t shooting a scene so there would be no need for blanks in the gun. As far as an actor or actresses not knowing the condition of each firearm they pick up, they could know not to point it at someone and ask “where are the ballistic shields?”.
They were rehearsing for a re-shoot. Apparently it was important to someone that the scene show the cocking of the hammer. Baldwin was asking “Can you see that?” as he worked the hammer. The people apparently would not be behind the camera, or would be behind shields if they were expecting blanks to be fired. They could as easily have asked where the shields were. Do you think they rehearse the scene and then take time to load blanks? The reason for an armorer and a secure chain of custody is to smooth out those problems. Time is money.
It would also have been safe if the armorer had loaded one, skipped one, and loaded four. The hammer would have been resting on an empty chamber. I don’t know how many fired shots the scene called for.
Just sounds like sloppiness and laziness all around, in the interest of saving time.