Bismillahir Rahmaanir Raheem.

It is not recommended to donate blood while fasting unless there is an absolute necessity to do so. Drawing a lot of blood may make a person weak and may force him or her to break the fast. So, indulging in such an action is not at all recommended as we are advised to keep our fasts intact as best as possible. Moreover, there are scholars who are of the belief that drawing lots of blood would invalidate the fast. This is the opinion of Imam Ahmad. However, Imam Shafi and others are of the view that drawing of blood (by itself) does not break the fast; it is still not advisable to do so as it may inadvertently cause a person to break his fast.
The ruling, however, is different when faced with an emergency. That is the case when one is called upon to donate blood in order to save another person's life. In this case, one must not have any hesitation to do so, even if he or she were forced to break their fast by doing so.
Another exception: If a person is exceptionally strong and is confident that donating blood would not render him weak, there is no sin for him in donating blood, as per the rulings of Imam Shafi and others. In conclusion, each person should consider his own state of health before embarking on this action. Allah knows best. May Allah forgive our shortcomings, heal our sick brothers and sisters and aid all muslims in any form oppression. Aameen.