Is it permissible in Hinduism to donate blood? I have no idea whether Hinduism ever talk about blood donation! Are there any slokas regarding this?
Why Hindu Dharma will stop someone from saving a life? Hindu Dharma promotes donation instead. To save a life, whatever sacrifices you have to make, that will be real dharma. And your those sacrifices will make you a pure soul or dharmatma. To save a life whatever one has to do that will be Dharma.
Now let's see some examples by which you will come to know that sacrifices are a necessary part of Dharma. Dharma can't be completed without sacrifice.
An ancestor of Shree Ram named as Maharaja Shibi sacrificed his whole body to save life of a bird.
Another ancestor of Shree Ram named as Raja Harish Chandra, who sold his own wife and children in open market just to save his promise (रघुकुल रीत सदा चली आयी, प्राण जाये पर वचन न जाये ).
- In Mahabharata, Karna did donate his life saving armour to Indra.
- It is stated in last lines of Satya Narayan katha that A vaishya named as Sadhu gets moksha by cut down (आरे से चीयर कर) his own Son with chainsaw.
- Bhagwaan Parshuram also chopped head of his own mother just to fulfill his promise given to his father.
Now what will you ask by seeing those examples? may be something like this, Is selling own family in market allowed in Hinduism or chopping head of own Mother is allowed in Hiduism? There are many more examples can be found in history...
Now think again what you are asking, You are asking Blood Donation is allowed on hinduism... Just see those examples and you will come to know blood donation is a very very small thing in front of those sacrifices which are being done by great Dharmatma by getting inspiration from Hindu Dharma.
You are worrying about blood donation but people have sacrificed their body, own wife and children and even mother just to save their Dharma.
Let me clear my points. As per Bhagavat Geeta, Dharma is totally depended upon situation. Hence Dharma changes every second. One karma(deeds) which is Dharma right now that same Karma can be a sin also in different time or situation.
For example: Telling lie is sin, but if that lie is being told to save someone then that sin is greater than truth said by Shree Krishna in Bhagwat Geeta.
Hence our karma itself can't be bad or good but instead our intention behind that karma makes our karma bad or good. If your intention is pure than no sin can be done with your hand. Whatever Karma (whether blood donation or anything else) you will do that karma can be sin(paap) or Saintly(punya) depends on situation. There is no such karma exist in this world who can always be Saintly or always be sin.
Hence those sacrifices which I have shown above must seem so cruel on first sight but those sacrifices are totally correct in point of view of Hindu Dharma. As I said, all karma depends on situation. Sometimes a situation comes in life when two dharma conflict with each other. For example.. In one of above example, one conflict was between saving a promise or saving a family. Then in that tricky situation (Dharma Sankat). One has to decide which Dharma is greater and he/she has to follow that Dharma by giving up on other. In above example saving a promise is more important than saving own family as per Hindu Dharma.