Enzymatic cleaners break down proteins that form stains. Since blood stains bond to the fabric using proteins, enzymatic cleaners can be highly effective at removing them. However, wool and silk fibres are made from proteins, and may break apart if exposed to an enzyme product. If you are having trouble finding a cleaning product labeled "enzymatic" or "enzyme cleaner," try a "natural" or "earth-friendly" laundry detergent or laundry pre-treatment, which often contain biodegradable enzymes. Nature's Miracle and Seventh Generation laundry detergent both fall in this category. Agitate the fabric with your fingers to help scrape off the crusted material, or scrape it off using a blunt knife. Dissolve about 120 mL (1/2 cup) of the cleaner in a bowl of cold water, then submerge the stained fabric. The soaking time will depend on how old the dried blood stain is, and how strong the cleaning product is. Soak at least one hour, or for as many as eight. Optionally, scrub the cleaner into the stain with a toothbrush before submerging. Wash the fabric as usual, but do not put it in the drier, which may cause the blood to permanently set. Let it air dry, then check whether the stain is still present.

TL;DR in Vietnamese:
Không nên sử dụng phương án này cho vải len hoặc tơ. Tìm kiếm loại nước tẩy rửa chứa enzym. Xối nước lạnh vào vải để làm máu khô nhả ra. Ngâm vải trong nước lạnh với nước tẩy rửa chứa enzym. Giặt miếng vải và phơi khô.