Yes, salah is valid in those clothes. Impure blood is one that comes out at the time of slaughtering an animal which is called dam masfooh (blood that pours forth). And the blood which is in the meat of the animal is not impure. If it smears the clothes then one can offer salah without washing it.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband