There’s no problem in donating blood while fasting, if you are physically fit and capable. And lots of Islamic Scholar have expressed similar opinions regarding this, including Allama Sheikh Yusuf Al Qardawi, Mufti Osman Shahin and many more.

According to Sheikh Yusuf Al Qardawi, someone can save a dying person by donating their blood, and that would be the best help s/he can offer. And according to mufti Osman Shahin, there’s no prohibition for donating blood as per Islamic Shariah, if that person is physically fit.

Maolana Ziaur Rahman and Maolana Mujahid Uddin- the two head muftis of Madrasah Darul Ulum of Mirpur, quoted Hadiths in this respect, which says-

Akrama (R) narrated, “The Prophet was cupped while he was in the state of lhram, and also while he was observing a fast.”

Thabit Al Banani narrated, “Anas bin Malik was asked whether they disliked the cupping for a fasting person. He replied in the negative and said, ‘Only if it causes weakness.’.” (Sahih Al-Bokhari 1:260)

This clearly implies that there’s no harm to your physical state if you donate blood during Ramadan, rather your rewards for saving someone’s life would be multiplied by 70 times for blessed month of Ramadan. And if you donate blood in Quantum Lab that would be multiplied by further 4 times, since the components of blood are separated into 4 parts that could be utilized to save 4 individuals than just one.  

Therefore, come and donate your blood without any further delay, if you are physical fit and capable and don’t deprive yourself from such a huge task of saving one’s life.