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.