I have some dried pig's blood (fresh is not at all easy to buy in my country), which I wish to use in recipes where fresh blood is called for. How much dried blood + water should I mix to replace e.g. 1 litre of fresh blood?
Reconstituted dried blood will not have the quite the same texture as fresh blood. If the dish you're making requires blood as a thickener -- particularly if it's supposed to have a thick, gelatinous texture -- dried blood may not work correctly. For other textures, though, dried should be fine. By way of example, nearly all black pudding is made with dried blood, but drisheen (an Irish blood sausage with a more gelatinous consistency, made without oats to thicken) needs fresh blood.
In terms of water content, roughly 4 parts water to 1 part dried blood (by weight, not volume) should be fine.