In case that there is no way to provide medical treatment to the patient other than blood transfusion and that, according to doctors, it is most probable that the patient will benefit from doing so, blood transfusion is considered permissible. Unlike breastfeeding, blood transfusion does not convert non-Mahram (marriageable relative) to be Mahram (unmarriageable relative). Allah (Exalted be He) says,
He has forbidden you only the Maitah (dead animals), and blood, and the flesh of swine, and that which is slaughtered as a sacrifice for others than Allâh (or has been slaughtered for idols, on which Allâh’s Name has not been mentioned while slaughtering). But if one is forced by necessity without wilful disobedience nor transgressing due limits, then there is no sin on him. Truly, Allâh is Oft-Forgiving, Most Merciful.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.