Donating blood does not invalidate the fast. This is because what nullifies the fast in principle is the entry of substances into the body, not the exit of substances from it.
It is reported that the Prophet (peace and blessings be upon him) underwent cupping (ḥijāmah) while he was fasting (al-Bukhārī, Ṣawm, 32; Abū Dāwūd, Ṣiyām, 29).