Is it considered to be a good charity to donate blood to someone in need. Is it permissible or is it encouraged in Islam? Also is it considered as a charity in Islam?
Some fatawa say blood donation is a rewardable act, such as:
... whoever donates his blood to save the lives of people, indeed, he gets a good reward for that if he does so for the sake of Allah
... there is nothing wrong with giving blood to him as an act of charity, and you will be rewarded for that ...
None of the fatawa I looked at contradicted these fatawa, in general. However, there were some precautions, such as the blood not being used for impermissible things (IslamWeb). See the linked answer for more stipulated conditions.