It is not correct to draw an analogy between donating blood and breastfeeding to prove that someone is a mahram
This analogy is not correct. The relationship of mahram that is proven in sharee’ah applies only in the case of breastfeeding. This has been stated by the consensus of the Fiqh Council.
Majallat al-Buhooth al-Islamiyyah, 35/343.
The Standing Committee was asked:
A man had blood taken from him to be given to his wife. Does that have any effect on his married life with her?
Perhaps the questioner is thinking of an analogy between blood and the milk that establishes the relationship of a mahram. This analogy is not valid for two reasons:
1- Blood is not a source of nourishment as milk is;
2- That which establishes the relationship of a mahram according to the text is breastfeeding that fulfils two conditions: (i) that the breastfeeding occurs five or more times; (ii) that that takes place within the first two years of the child’s life.
Based on this, the blood that has been taken from you and given to your wife does not have any affect on your married life. And Allaah is the Source of strength.
Majallat al-Buhooth al-Islamiyyah, 4/332.