Black oxide is simply an oxide of iron, also known as magnetite.
Magnetite dust is considered a nuisance particulate, and its concentration in air is regulated by OSHA. The safe limit is 10mg per
cubic meter in air.
As for toxicity by ingestion, this is taken from the Material Safety Data
Sheet (MSDS) for magnetite:
"May cause severe and permanent damage to the digestive tract. May cause liver
damage. Causes severe pain, nausea, vomiting, diarrhea and shock. May cause
hemorrhaging of the digestive tract.
Note: The toxological properties of this substance have not been fully
So basically, you don't want to breathe its dust, and you don't want to