Answered by Shaykh Abdul-Karim Yahya, SunniPath Academy Teacher
I understand that blood on one’s person, if it is a result from one’s own doing, is only excusable (for prayer) if it is (customarily) small.
Question: if one scratches one’s self and it bleeds thereby, is the resultant blood on one’s person “a result of one’s own doing”? Or does this expression mean that one has to intend the releasing of one’s blood in such a manner?
In The Name of Allah, Most Merciful and Compassionate
A large amount of one’s own blood on the body or clothing is excused in prayer with three conditions:
(1) that it not move beyond the place it naturally reaches when oozing from the wound (even if it flowed from a wound on the head all the way to the foot before stopping);
(2) that it not be by one’s deliberate action; and
(3) that it not mix with something foreign (which is something that there is no need for it to contact).
If either of the first two conditions are not fulfilled, only an amount of blood that is commonly acknowledged to be small (meaning it is difficult to avoid) is excused.
If the third condition is violated no amount is excused.
To answer your question, a large amount of blood which resulted from accidentally scratching one’s self is excused. This is because the meaning of: “a result of one’s own doing” is that it be the result of one’s intentional action. This is a translation of the way Ba’ashin phrased this condition, and is what is understood from Shaykh Ibn Hajar’s, Allah have mercy on him, discussion of excusable blood in Tuhfah. And Allah knows best and He alone gives success (tawfiq).
 Summarized from: Sa’eed Bin Muhammad Ba’ashin, Bushra al-Kareem (Beirut, Dar al-Fikr, 1412AH/1992AD), pp. 199-200.