Blood May Not Be Compared With Milk
Regarding the Spreading of Prohibition

Question :

If a woman required blood and it was taken from a person unrelated to her, then she became well and that person wished to marry her, is it permissible?

Answer:

Yes. It is permissible for a person to marry a woman who has been given his blood because blood is not (the same as) milk, that we might say that it prohibits (marriage). What prohibits (marriage) is milk on condition that it takes place in the first two years, before weaning occurs. And it has been confirmed from the Prophet that he said:
"What is unlawful by reason of lineage is unlawful by reason of fostering."

Question :

The following question was received by the General Presidency for the Administrations of Scientific Re- search, Religious Verdicts, Preaching and Guidance: A man says that his wife was ill and her condition necessitated a blood transfusion, and the hospital took blood from him for his wife, so he is asking if this will affect his married life with her.

Answer:

The Permanent Committee for Scientific Research and Religious Verdict answered as follows: It seems probable that the questioner has in his mind some analogy between blood and milk, which causes prohibition (of marriage between foster brother and sister) and this analogy is incorrect for two reasons: The first is that blood is not a nutrient like milk. The second is that what causes prohibition according to the texts (of the Qur'an and Sunnah) is breast-feeding with milk on two conditions:

The first is that the breast-feeding is repeated five times or more. The second condition is that it takes place during the first two years. In accordance with this, there is therefore no effect on your married life with your wife from the blood which was taken from you to her. And Allah is the Granter of success. And may peace and blessings be upon our Prophet, Muhammad and upon his family and Companions.

Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 5 Pages 220-221-222