Blood Does Not Transmit Prohibition Like Breast-feeding
Question :
If a woman became ill and required blood and it was taken for her from a person who was unrelated (i.e. not a Mahram) to her, then Allah, Most High cured her and that person wished to marry her; is it permissible or not?
Answer:
The case of taking blood from the man for the woman, and it being transfused to her in order to give her strength does not result in a prohibition like breast-feeding, even if it was a lot. The ruling is the same if a man is transfused with blood. from a woman. Therefore, it is permissible for each of them to marry the other.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 6 Pages 137-138