Is Blood Najis?
Question

Ash-Shawkani says: Impure substances include: 1. The stools and urine of humans, except the urine of a male infant; 2. The saliva of a dog; 3. The stools of animals that cannot be eaten; 4. Menstrual blood; 5. The flesh of swine. Anything other than these is not najis (impure), even if people regard it as filthy, because there is no evidence for it being haram in the Quran or hadiths, except for eating it. 
My question is: what is the evidence that the blood of humans and other animals, alive or dead, is impure? What is the correct view regarding this matter, because the supporters of every view claim that their view is derived from the Quran and Sunnah? Which view should I follow?
Praise be to Allah.Is blood impure?
Flowing blood is impure according to scholarly consensus. That is indicated by clear evidence from the Quran and Sunnah, including the verse in which Allah, may He be glorified and exalted, says (interpretation of the meaning):
{Say, “I do not find within that which was revealed to me [anything] forbidden to one who would eat it unless it be a dead animal or blood spilled out or the flesh of swine - for indeed, it is impure - or it be [that slaughtered in] disobedience, dedicated to other than Allah. But whoever is forced [by necessity], neither desiring [it] nor transgressing [its limit], then indeed, your Lord is Forgiving and Merciful”} [Al-An ‘am 6:145]
Imam at-Tabari (may Allah have mercy on him) said:
“Rijs (translated here as impure) is that which is najis and putrid." (Jami‘ al-Bayan  8/53)
Evidence that blood is impure
With regard to evidence from the sahih Sunnah, it was narrated that Asma bint Abi Bakr (may Allah be pleased with her) said: A woman came to the Prophet (blessings and peace of Allah be upon him) and said: Menstrual blood may get onto the clothes of any one of us; what should she do with it? He said: “She should scratch it, then rub it with water, then wash it, then pray in it.” (Narrated by al-Bukhari (227) and Muslim (291)
Al-Bukhari included this hadith in a chapter entitled: Chapter on washing blood. An-Nawawi included it in a chapter entitled: Chapter on the impurity of blood and how to wash it. Even though this hadith speaks of menstrual blood, there is no difference between one type of blood and another; all blood is of the same nature, no matter where it comes from.
There is no difference of opinion concerning this ruling among the scholars of the Companions, the Successors and the four imams.
Imam Ahmad was asked about blood, and it was said to him: Are pus and blood the same in your view ?
He said: “The scholars did not differ concerning blood, but they differed concerning pus ." (Sharh ‘Umdat al-Fiqh by Ibn Taymiyah  1/105).
Imam an-Nawawi (may Allah have mercy on him) said:
There is a great deal of evidence that blood is impure , and I do not know of any difference of opinion concerning that among the Muslims, except what the author of al-Hawi narrated from one of the scholars of kalam, who said that it is pure. But the view of the scholars of kalam does not carry weight when there is consensus and their view differs from the correct view, which is that of the majority of the scholars of usul among our companions and others, especially with regard to matters of fiqh." (Al-Majmu‘  2/576)
A large number of scholars narrated that there is scholarly consensus that all kinds of blood are impure . We have mentioned Imam Ahmad and an-Nawawi above. These scholars also include: Ibn Hazm in Maratib al-Ijma' (p. 19); Ibn ‘Abd al-Barr in at-Tamhid (22/230); al-Qurtubi in al-Jami‘ li Ahkam al-Quran (2/210); Ibn Rushd in Bidayat al-Mujtahid (1/79); Ibn Hajar in Fath al-Bari (1/352); and others.
It is better – according to Islamic teachings and reason – to follow this view, which the scholars narrated at the level of tawatur (meaning that it was narrated by so many from so many that it is inconceivable that they could all have agreed upon a lie), and accept it. It is a view that is based on clear texts of the Quran and Sunnah. The statement of ash-Shawkani and those who followed him in the view that blood is pure is outweighed and is contrary to the evidence and scholarly consensus. You should not let this view cause confusion, and it is not permissible to think that the scholars could reach consensus concerning some matter with no clear, sound evidence, as some seekers of knowledge think regarding the impurity of blood and other issues.
And Allah knows best.