Yo I spent a year working in the UK blood donation service and just want to add some things to reassure people.
Blood is always checked. Comprehensively. It doesn't matter if it's someones first donation or their hundredth. There are multiple checks before you even donate, and we send samples out to get them tested for any blood borne diseases long before the blood makes it's way into anyone. We sort the blood by group ourselves and it's always at least triple-checked on our end. Then it's checked again at the storage site, and once again in hospitals. We all have access to a massive, comprehensive database with every single donor. The bags are clearly labelled and barcoded.
If there was a single fatality linked to a blood donation, there'd be a massive outcry, and an even more massive internal reaction. Part of why the UK system is so careful is because of the mad cow outbreak, which was related to transfusions and honestly still gave some of my coworkers flashbacks. Everyone is constantly guarding against anything like that ever happening again.
The only significant difference between "male" and "female" blood is very slightly different haemoglobin levels. Donated blood with a different haemonglobin level will not kill you. It's meant to be higher than yours, because thats part of the fucking point of blood donations. Higher haemoglobin helps people your blood carry oxygen and helps your body with basically every vital function. Which is why we test haemoglobin levels on site and only accept donations from people with higher than average.
So yeah, one single difference. You know what this incredibly risk-averse, comprehensively research-led system said how trans people might affect that? That they don't. Our full training was literally "treat them like their identified gender"
So no, a donation from someone with the "opposite" sex won't kill you. You know what will kill you? Propaganda like this.
We have literal evidence in living memory of what happens when mass hysteria is whipped up over blood donations. My parents lost one of their best friends in Mexico during the AIDs crisis, because the media wouldn't stop screaming about contaminated blood and how it would kill you and your entire family. He died in his thirties from a fucking nosebleed.
JKR cares more about clinging to her transphobia than she does about the very real deaths that could come from this. She cares more about clinging to relevancy than she cares about the lives of the people who somehow still see her as worth listening to. Her morals are a joke on a cosmic level and I hope she rots.