If he couldn't defend against it, and he wasn't instantly killed mind you how are Heralds going to be untouched? If it nicks them they die, if it hit's their blade it will begin to break it, this is something they have not dealt with before, and Nightblood has other properties, even mostly sheathed Szeth broke a man's back with it in a single hit.
Easy fix
It takes seconds not minuets for Nightblood to Force Suicide.
Where? The only ones we see with it are Leshwi, Raboniel, and El, Some of the most important Fused we know of.
Like what?
If they touched the blade after any substancial period of Time out of the Sheath forget the Suicide effect it will consume their souls, they don't hold enough Voidlight to sate it.
And how would using the sheath to block it let them know that turns it off? For all they know it's just ordinary Aluminum.
But he knew about Nightblood, and for a year couldn't do anything about it.