In my family czernina has been traditionally made only with duck or goose blood -- both very good. However, I have heard that in a pinch pork or rabbit blood may be used, but NEVER chicken blood! I have always wondered why, but never decided to actually try it. Why spoil a whole pot of soup just to prove a point, eh?!
I can remember that my grandparents would only use Muschovy ducks. My grandfather would either kill and bleed them himself or watch as the butcher bled them to be sure he only got the duck blood and not chicken or pig blood mixed in. I don't think I ever asked why, but I sure do love good czarnina!