Giveaway creator blacklisted me after I won their giveaway

the blacklist doesn't mean anything for your win, just wait patiently for your key. It just means you won't join their GA's in the future.

As long as you receive a gift it should be okay, maybe creator just making sure people who won before don't participate again?

I would wait a few days before reaching to support, but then yeah, that would be best way to resolve it since you probably can't reach creator being blacklisted

You will get the key to the game if you have already won. Don't worry

just wait until you get the key or the 7 day period is over. if you havent received anything until then, then simply mark as not-received.
if you received a dupe key, then you can try to contact the creator through other means.
e.g. an old discussion or post on the forums. i guess in these cases ppl will forgive you for necroing. or through steam itself, like posting on their profile. but be warned that youre not allowed to write certain words like "won/win" or "gift". it might trigger steams algorithm an ban your accout.

no need to contact support

It happened to me too. It shouldn't have any impact on this particular GA, but you won't be able to enter any other of their GAs. If you ask me, the reason could be your not-so-stellar ratio of sent/won. I don't judge, but I understand people who do.

I don’t know if this is still a thing, but years back I remember some users would blacklist anyone who won their GAs so that they couldn’t win again. They can still send you the key and you can still activate it :)

or if there’s a reason for the blacklist

It's just a guess, but I could imagine your giveaway creator took a look at your (real) CV ratio