Pork (pig meat) is forbidden as many times in the Quran as blood is (2:173, 5:3, 6:145, 16:115). Under that, I'd assume that blood is just as forbidden as pork.
However, Muslims are very picky about whether food contains traces of pig DNA or pig hairs (which is not explicitly forbidden in the Quran). They are not as picky with something such as blood, even though quite a lot of blood is commonly found in halal food.
Why is this? Is there a hadith that emphasizes how forbidden pork is? Is there something detailing that blood is allowed in smaller quantities?