User blacklisted me for not wanting to redeem a trojan/virus game. Is that reasonable?

As others have stated, technically you're in the wrong as you won a game on SG and not activated it. Rerolling is allowed and for the reasons stated, I think that is fair. However, blacklisting can be done for way less reasons that this so that's up to the giveaway creator if he/she wants to be a douche or not. Being on a blacklist is very common these days on sg so don't worry about it.

I think the definition of a trojan is being mistreated here...
That being said, I don't see a problem with refusing a game as long as you haven't revealed the key. Reroll requests are easy and quick to send, and are processed pretty quick too. And it gives a chance for the game to be won by someone who will use it.
If the key was revealed though, I can understand the GA creator being annoyed. I mean, I once had a case where someone who requested a reroll had actually used the key but claimed they already owned the game...

You are a lucky person! Thanks to that blacklist - you won't need to deal with that idiot in the future :D
If somebody would tell me that game I'm giving away is pseudo-game virus - I would rather delete that GA.

My question to you what was the big deal? You could have simply redeemed the key. Just by adding the game to your library doesn't give you a virus :D
No one forced you to play the game

It's entirely reasonable to BL people asking for rerolls for games they don't own. It's just less trouble down the road.

I understand if they got it some other way and already have it, but not because people are randomly joining and not wanting it after all...

it's up to the winner to accept or decline a gift, for whatever reason.

I find it really hard to decide who is right and who're wrong in this matter.
The giveaway creator has a point, I personally would also feel that "I don't want the game" isn't a valid reason for a reroll. On the other hand, I can see where you're coming from - it shouldn't be YOUR responsibility to make sure that a game isn't a crypto-mining virus, that should be Valve's responsibility. Interesting situation.

In this particular case, if I was the GA creator, I would most likely have asked support to delete my giveaway instead of asking for a reroll and possibly infecting some poor guy's PC.

There was no miner or malware, just a lot of misinformation out there that's been explained well here: https://www.reddit.com/r/Games/comments/937e7m/abstractism_has_been_removed_from_the_steam_store/e3dke8s/