Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
God will definitely reward the person who makes blood donation to save another’s life either by raising his rank or by forgiving him his bad deeds.
Answering your question, Dar Al-Ifta Al-Misriyyah, states:
God will definitely reward the person who makes blood donation to save another’s life either by raising his rank or by forgiving him his bad deeds. God Almighty says, {Is there any reward for goodness except goodness?} (Ar-Rahman 55:60)
The Prophet Muhammad (peace and blessings be upon him) said, “Muslims are brothers to one another. God meets the need of whoever meets the need of a fellow Muslim. A Muslim does not oppress a fellow Muslim nor does he hand him over to an oppressor. Whoever relieves the burden from a fellow Muslim, God will relieve him of one of the burdens of the Day of Judgment. And whoever covers [a fault] for a Muslim, God will cover for him on the Day of Judgment.” (Al-Bukhari)
The Prophet also said, “Whoever relieves the distress of a Muslim in this life, God will relieve him of a distress on the Day of Judgment. Whoever facilitates a Muslim’s difficulty, God will facilitate his difficulties in this world and in the Hereafter. And whoever conceals the faults of a Muslim, God will conceal his faults in this world and in the Hereafter. God will help whoever helps a fellow Muslim.” (Muslim)
Scholars maintain that in order to make blood donation, the donor must be healthy and legally competent.
Almighty Allah knows best.
Source: Egypt’s Dar Al-Ifta