Two factors to consider id say. Blood isn't the only organic source of nitrogen so it's not as if its necessary, thus I'd wager many vegans would consider it unnecessary animal suffering, at least in theory. However the caveat, and second factor, would be blood is byproduct, no ones killing the animals in order to obtain blood meal so many people including vegans may think it more ethical to not let it go to waste since weather or not there's a demand for blood meal, there will still be animal blood that needs to be disposed of.
Strictly dietarialy, yes they would still be vegan. All soil is full of countless formless decomposed animals and plants, it's an inescapable reality of how the soil came to be. It can only get more ethically involved when you choose to add it yourself imo.
That second point would require intimate knowledge about which animal parts would be disposed of if they didn't find a buyer.
In reality, everything is used. If there wasn't a market for part of an animal, a use was found and a market created (which is part of the reason why industrially produced white sugar, beer, wine, apple juice, potato chips and bread usually aren't vegan).
Anyway, vegans usually don't care about whether an animal product could be leftover. Their philosophy boils down to "Just fucking leave animals in peace."
Their philosophy boils down to “Just fucking leave animals in peace.”
It's more complicated than that unless you don't understand how many animals die when you clear farmland. Every crop you eat came at a cost to animals, if there's no amount you deem acceptable or unavoidable your only option is to exclusively eat food you grew yourself, and that still alters the environment to be less favorable to animals, you just don't directly kill them like large scale farms do.
Even honey isn't okay with some (I have no idea the %, could be most or just a small number) of vegans. So regardless of how the blood was obtained, there is at least some who would not consider it vegan.
There are vegan blood meal alternatives out there to resolve this exact conundrum.
But the reality is, unless your plants are being grown hydroponically in a sealed warehouse or similar, chances are real good that they are feeding on decaying animals (either directly or indirectly) whether you like it or not. They're mostly insects and annelids and such, but still animals.
I think the issue for vegans is more about whether animal slaughter was involved in making their fertilizer. Dead pillbugs in the soil is just nature doing its cycle of life thing.
Its easiest to treat paying for something the same as doing it firsthand.
It gets really strange to find the line that separates how far away from an immoral act you need to be to be considered moral still. In the same room? In the same building? What if you don't explicitly ask someone to do the immoral thing, and only ask for the remains of it?
Some indigenous peoples cooperate with their natural environment. Humans are fundamentally a keystone species that's collectively gotten really bad at it, to get good at other things. We could have human conventions, art, and technology that works entirely with nature and our environment rather than against it. Between these facts, I'm not a fan of that definition.
Rest assured, i don't agree with it either, but as you say this seems to follow from the statement
We shouldn't treat/cure cancer, cancer happens in nature and we're a part of nature
We shouldn't try to prevent rape, rape happens in nature and we're part of nature
We shouldn't try to limit animal suffering, animal suffering happens in nature and we're part of nature