I think the winner of my first giveaway blocked me instead of adding me as a friend on steam.

Try to contact them by leaving a comment on any of their giveaways here explaining that, if they refuse to add you after a week, you can contact support and ask for a reroll, since the winner would effectively refuse to accept the gift.

The e-mail used to be an option because you could buy a Steam gift and send it trough e-mail, but you can't do that anymore, you could try writing to contact them there, but I think a comment here is more effective.

This has happened to me once, but from the opposite side. After realizing what I was doing, I unblocked the sender and apologized to him.

And a tip if you allow me, if in a future giveaway the winner must accept a steam friend request, it is good to mention this in the description.

...and the Epic Blunder.

Seems rather befitting, given the circumstance.

Ok guys, everything is back in order, and they received the gift.
Thank you for your help. ♥