The World Federation One Stop Fiqh

Ruling 842

If blood spills on clothing that does not have a lining and it reaches the other side, it is considered to be one blood; and the side on which the blood has spread more must be taken into account [when determining its size in relation to a dirham]. However, if the other side of the clothing becomes bloody separately, then each side must be considered separately. Therefore, if the combined area of blood on the front and back of the clothing is less than a dirham, prayers with it are valid; but if it is equal to or more than the size of a dirham, then prayers performed with it are invalid.