For me it only didn't work if I blessed the blood after initializing the flow... cause if you let it flow first and then bless the first blood that reaches the bowl will be the one that wasn't yet blessed... logic right? You need to bless first and then activate the flow. Don't worry that the blood at the floor will loose its blessed status after 2 turns, for me it worked just fine with only 1 bless, you just need to bless first and then activate the altar.
As for getting blood i just exploded some bloated corpses raised from skeletons around.
