I would like to know how to thicken sauces with blood and is it safe? I saw a recipe (French) that involved thickening with blood (they also said puréed liver would work) - is this safe?  I've eaten blood sausage and black pudding, but still am curious about a sauce?
It should be safe, you would want to follow the same precautions you would when preparing meat. I personally like thickening my stews with bone marrow.