So, after experimenting after looking at this, the way to get blood is as follows.
1. Put somthing in the smeltery, wait for it to smelt.
2. Put yourself or a mob into the hole in the center
3. As you or themob takes 5 damage, the smeltery will collect 5mb of blood
I suggest that you put an iron ingot and an emerald to smelt, as that plus 80mb of blood will produce a pig iron ingot