You can CTRL-click the blood to be the bottom fluid in the smeltery, drain it into a vat, then remove and replace the vat. This way you don't lose any metal in the smeltery.
[Edit] After reading MacAisling's post below I realized I meant to say tank instead of vat.
I've seen Emerald smelters use blood as the fluid to kill villagers then pump it out of the smelter using a fluiduct network and pneumatic filters to whitelist the blood into a tank and the metals to separate basins to make blocks while cycling the fluids continually.
It goes without saying that metals that combine into alloys were separated into different smelters, unless the goal was to obtain the alloys.
Now IIRC, it won't pour into a casting basin (no blood blocks to make) but will pour into an empty casting table to make blood drops (ore dictionary as slimeballs), which can also be dried on a drying rack & eaten. You can also pipe it into a tank to keep it in liquid form. Put horses in the smeltery & you will get glue (another slimeball alternative). Villagers will get you emerald.