The best way to do a Blood Smeltery is to put a Nether Portal with one side that fall into the smeltery and the back side being a wall. Then you close the sourroudning or make it so no creeper can come near it as they grief Seared Objects.
Edit: Any object falling into the smelter that is above liquid will float. If no liquid is in the smeltery, items will be in the smeltering grid. But if there's no liquid, monsters won't be hurt by the smelter.
edit #2: Completely off-topic, i just noticed i ranked up to "Member". I'm gonna go celebrate
I haven't thought of a Nether Portal! The possibilities!