An 'enchanted' metal formed from a dark ritual.
Dark Iron is a normal gun metal grey, however, it radiates 'darkness' from the point of view of anyone. This gives it the impression of being darker than it actually is, practically black.
Dark Iron is normal steel that has undergone a process through a 'Blood Magic' ritual, causing it to darken and increase in durability. Dark Iron, after being shaped, typically reforms to its proper shape. Machine built of this substance, seem to 'regenerate'.