won --> activated --> bl ??

Blacklist him/her too /s

For real, just mark as received and move on.

Some people like to do that to eliminate duplicate winners, nothing personal

I think you have send too many Group giveaway (almost low entries) , and win more public giveaways .

so he dont want u join his giveaway again .

there too many user use Small Group/ Invite giveaways for Level up, then go to win High level public giveaways .

Did u think it fair ?

thx - i only want to thank for the win (i always do) - but how?

Maybe he just jealous with your luck, dude, u got insane wins there on 1v3000, 8000, Jesus!
U really are son of the b̶e̶a̶c̶h̶ luck xD

go for the easiest explanation: ga creator doesn't want you to win from them again.
you don't need to thank that person in a comment. if they expected you to thank them, they wouldn't have blocked you. 👍

Did you activate it immediately? I, for example, blacklist someone if i have to beg them to activate their win although they seem online everyday (bot?).

I had that case one time too.
You can do nothing to thank him/her if he/she don't have a open comment function at the steam profile.
If it is important for you, you are able to look in the discussions for a comment of this person and answer to it but i would not expect a big reaction from that person. So i would advice you to save your time and move on.

It happened to me too, once.
Supposedly it's to prevent you from winning another giveaway by the same person.
As if the odds were high already!

If one of your friends also uses this page, just make him post that reply for you.

Probably due to your negative ratio (some people expect a ratio close to 1:1) - best to just mark as received and move on, since they've already blacklisted you and there's nothing you can do about that.