pukunui
Legend
@Bruinlord: Just FYI you have necro'd a thread from 2017 and are replying to someone who's been banned from the site so won't be able to respond to your comment.
I think it is pretty clear that you can do it if you cast shadowblade when you finish your long rest and choose it then. You can't choose it later in the day and there is good reason for this. Whether it is still your pact weapon when you cast it again is debatable.So let's say I wake up and don't choose a weapon. Then, later on, I get into a fight and summon a Shadow Blade. Then I go to myself, "Yah know what, I'm touching this weapon!"
Would you then get to add the benefits of the Hex Warrior ability?
Given that it automatically applies to any Pact Weapon you summon, it's not like you'd be without a weapon during the rest of the day either, even after the Shadow Blade has disappeared.