Hi, Sally- the one way to tell whether or not the metallic smell in the manure is due to blood is to test for occult blood in the manure.  You can use the Succeed test, or take manure to your vet to get it sent to the lab for occult blood testing.

It is fine to post this here, since it is dealing with your IR mare.

Good nose on you!  If she is passing blood from somewhere, it is best to find out.