Bismillahir Rahmanir Rahim
Wa'laykum-as-Salaam-wa Rahmatullahi wa Barakaatuhu
This question, is quite a controversial question, as there are many different legal opinions relating to it. However, I will try to attempt to answer the question in connection of the Ahl-as-Sunnah wal Jamaa'ah, their scholars, and opinions.
As far as giving blood is concerned, then blood, is that which circulates the body, without which our lives would come to an end. It is our duty to ensure that we look after our bodies, and keep our blood clean, free from all those items which may cause disease, for example, diabetes.
As far as the blood itself is concerned, then according to fiqh, it is regarded as an impurity. It is also one of those things, if it flows after being cut or injured, that will make ablution invalid.
In the Noble Qur'an, the word blood (damm), has appeared in many places, especially, in those verses, which expressly state the prohibition (hurmat) of something. For example:
'Hurrimat 'alaykumul maytata wad damma wa lahmal khinziri''We have made haraam upon you, the dead, the blood, and the meat of the swine'.
This ayah basically tells you that the listed things (dead animals, blood, and swine), are all forbidden. Here the mufassirin, have taken damm, to mean killing, because when you kill something this then entails blood. Therefore, it is haraam to eat something from which the blood has already flown, and is haraam to eat.
On the other hand there is a hadith-ash-Sharif of the Messenger of Allah (peace and blessings be upon Him) in Sahih al Bukhari, in the twenty fifth chapter, and other places in this book, which states:
'fa inna dimaa-ukum wa amwaa lukum wa a'raadu kum baynakum Haraamunn''Your blood, wealth, and honour are sanctified towards each other, just as this day'.
So on the one hand you have the prohibition from the Qur'an, about dead animals from which blood has been flown to be haraam, and in the Hadith -e-Mubaarikah you have the sanctity of the Muslims blood, wealth, and honour.
For this type of sanctity to remain in place one must fully respect and honour, the blood, wealth, and honour of another Muslims as they are sanctified. This will prevent us from giving our own blood in a cause which is not as necessary as it may seem. However, like I mentioned above, there are scholars ('ulama') who have said that blood may be given in any event to prevent death. Then there is the issue of Muslims donating blood to Muslims, yet the opinion that greatly influences, and makes sense, not only scientifically, but also commonly, is that where a relative for example a father, mother, brother or sister, are severly ill, are in hospital, or close to death, and they require blood, then to enable the sanctity of the family/Muslim, is to help him/her in whatever case necessary, in order to prevent death from occuring. This is the moral from the above mentioned hadith.
First of all, your families blood is the closest to you, and the dearest to you, In cases of emergency blood may be given. Secondly, it will prevent an outsider from giving blood where the blood code is similar to that which is in need. Thirdly, you may be saving a life which will carry reward in itself. Fourthly, the blood of a Muslim will remain the blood of a Muslim, as long as it is not wasted, thrown away, or disposed of.
Blood may also be given for the sake of medical tests, in order to identify a disorder which may be hereditary or non hereditary. Again, this may allow you to prevent death, or cure a disease, in which Islam perfectly deems permissable. However I do not think that giving excess blood, or blood when and to whom you feel like, is necessary, as this wiil render abuse, which will then render the sanctification of a Muslim to deteriorate.
May Allah the Almighty help us to help one another in the best of manners.Wallaho a'lamu biththawaab
M. Waseem Ashrafi