What it is and where it comes from
Flowing blood is forbidden in the Quran (2:173, 6:145). This covers foods made from blood, such as black pudding, and blood plasma or blood proteins used to bind processed meat. The blood that stays in meat after a proper slaughter is not counted.
Where you find it
Black pudding and blood sausage, some Asian soups and dishes, and some processed meat products.
What to look for on the label
Look for blood, plasma, blood protein or haemoglobin.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.