Do the people of Thedas make that distinction? Seems to me like it being blood magic in principle is what people react to, they don't care how it's used. It's like child molestation on 12 year old instead of a 9 year old to the people of Thedas, they don't care about the specifics because both are equally horrible to them. I personally agree that there is a distinction between the joining and blood sacrificing people, but were talking about Thedas people who are often quite stupid and reactionary when it comes to magic.
The answer is teaching them not to be, as I recently told someone in the third game. By the time Origins is over, your small party has accomplished a hell of a lot: you've chosen the new rulers for two kingdoms, earned the gratitude of a number of nobles and of everybody in Redcliffe and Denerim, and if you survive are honoured in a big ceremony where you get to ask a favour of the new king or queen.
The first time I did this, it was as an elven mage. Considering the legendary status that the Hero of Ferelden gains, I'd think that it would make lots of people reconsider their negative beliefs about magic and about elves.
If one person, with help from some friends, can accomplish that much, think of what many people working together could accomplish.
Anyway, it's been something like 3 years since I last played the first game, so I don't remember exactly how many hints were dropped and how subtle or blatant they were. But you say that one of them is that Alistair won't tell you--sometimes, in games like this, if somebody doesn't tell you something you just have to resign yourself to finding out later on. Which, as best as I can recall, is what I did.
I'll YouTube that mabari dialogue...hang on...
All right, here's what he talks about, in this order:
-Asks you to muzzle the mabari who swallowed darkspawn blood in a fight and is sick;
-Mentions that Wardens are immune to the taint;
-Asks you to get a flower in the wilds for him, which will hopefully improve the dog's condition;
-There are lots of dialogue choices here, but when you ask if the hound will be all right without the flower, he tells you that it'll probably need to be put down;
-The person who uploaded it didn't ask everything, like they left "Do you think I have nothing better to do?" unasked, probably because they didn't want to play the character as a ******.
The kennel master didn't say anything to this particular player about mabari surviving and becoming immune to the taint. Maybe another NPC did, but the LP I found is very long and I'm not gonna bother to watch all of Ostagar looking for the relevant dialogue.
Before I found that video, though, I found another, short video showing the resolution of that quest. You bring the flower back to the kennel master, and all he does is thank you and ask you if 20 silver will be enough for your trouble.
This is something that would be clearer on subsequent playthroughs, what TV Tropes calls a Rewatch Bonus. But the first time? Not really, not for everybody, and I think part of the reason is that a lot of people don't even consider the idea that you'll have to drink this stuff which you have been hearing is ultra-deadly. It just seems too crazy.