Firstly: 

“Blood poured forth” (6:145) is najis (impure) according to scholarly opinion. This has been discussed previously in the answer to question no. 114018. “Blood poured forth” is that which comes out of the animal at the time of slaughter. 

As for the blood that is mixed with the meat or that remains in the veins, it is not called “blood poured forth”. 

Secondly: 

What is required is to purify the clothes on which there is blood, or to change them, before starting to pray, because one of the conditions of prayer being valid is the removal of impurities from one’s garments or body or from the place where one prays, because the Prophet (blessings and peace of Allah be upon him) said: “If one of you comes to the mosque, let him look, and if he sees any dirt or impurity on his shoes, let him wipe it off and pray in them.”

Narrated by Abu Dawood (650); classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

But most of the scholars (may Allah have mercy on them) restricted that to cases where there is a lot of the impurity; if there is only a little you do not have to wash it, because it is too difficult to avoid it. 

It says in Mukhtasar Khaleel wa Sharhuhu: 

A spot of actual blood less than the size of a dirham is overlooked blood traces (of impurities) may be overlooked even if they are larger than the size of a dirham. This applies whether it is blood from menses, nifaas (postpartum bleeding), an animal that died naturally or a pig, from the body or from outside of it, on his own garment or the garment of another or on his body, in prayer or otherwise.

Sharh Mukhtasar al-Khaleel by al-Kharashi, 1/107 

Al-Hajaawi (may Allah have mercy on him) said: In the case of things other than liquids or food that are to be drunk or eaten, a small amount of najis blood from animals that are taahir (pure, i.e., permissible to eat) may be pardoned. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Pardoned means that it is overlooked and tolerated. Liquids refers to things such as water, milk and broth. Food refers to things that are eaten such as bread and the like. 

What is referred to with regard to things other than these two, such as garments, bodies, furniture, the ground and so on, is that a small amount of najis blood may be overlooked.

End quote from ash-Sharh al-Mumti‘, 1/439 

More than one of the fuqaha’ have clearly stated that butchers and others who carry out similar professions may be excused for small amounts of impurities that get onto their bodies and clothes because of their work, especially if they try hard to avoid that and remove whatever gets onto them of it, or it is too difficult for them to change their clothes in order to pray. 

It says in Sharh Mukhtasar Khaleel by al-Dardeer (may Allah have mercy on him): 

If she (a mother with a small infant) tries hard to avoid urine or stools by removing it as soon as (the baby) urinates, or by wrapping a cloth (such as a diaper) around him that will prevent it reaching her, then if anything gets on her after taking precautions, it may be overlooked. But it cannot be overlooked if she did not take precautions. The same applies to the lavatory cleaner and butcher. Ad-Dasooqi (may Allah have mercy on him) said in his Haashiyah: 

The words “and similarly the lavatory cleaner” refer to the one who empties the tank of waste. And in the case of (the lavatory cleaner and) the butcher who slaughters animals, they may be excused for whatever (impurities) get onto them provided they take precautions; if they did not take precautions then they cannot be excused. They have to wash off the impurity if they are certain or think it likely that it got onto them, but they may sprinkle water on it if they are uncertain. The words “the same ruling applies to other people in similar situations” mean situations similar to those of the lavatory cleaner and the butcher. The words “because it is unavoidable for them” mean that it is not completely under their control and they cannot avoid the impurities altogether.

End quote from ash-Sharh al-Kabeer wa Haashiyat ad-Dasooqi ‘alayhi, 1/72 

Al-‘Allaamah Ibn ‘Aabideen (may Allah have mercy on him) said:  

A similar case to urine is blood on the clothing of the butcher. The reason why only the butcher is mentioned is that in the case of anyone other than the butcher it is not to be overlooked, because the reason for the concession in his case is a necessity and that is not a necessity for others.

End quote from Radd al-Muhtaar, 1/322; see also 1/324. See also al-Mawsoo‘ah al-Fiqhiyyah, 40/113 

With regard to what is regarded as a small amount or large amount, reference should be made to custom. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: What matters is what the people think; whatever they regard as a lot is a lot and whatever they regard as a little is a little. 

End quote from ash-Sharh al-Mumti‘, 1/272 

And Allah knows best.