Wrong key to the wrong giveaway.

Nope, your stuck giving him two keys. You can't refund game keys on steam so nothing can be done

*edit your edit has a chance. Might as well try. I would make sure to mention in the ticket that both you and the winner are okay with that change

If they agree I think the giveaway could be deleted but I think the only way to really fix it is to give the winner the game of the giveaway, in this case Blackguards 2.

There really is not. He can't see on the key what game it is prior to redeeming it, so he isn't to blame. It's just a sad mixup, and my best suggestion is contacting support yourself, to see how to dismantle the situation.

Of course you can also just give him Blackguards 2 on top of it, and write off your losses. I know you didn't mean to do anything bad, and this isn't meant as an attack in any way, but the fault here really is all yours =/

yes there is an option to ask support to change the giveaway game, if the receiver is ok with that option. they will need him to post in the giveaway comments stating he is good with that.