Blackwell and Pangborn put the flunk in flunkies. They seem pretty intimidating at first. Jace believes they killed his father. But it turns out they didn't. They work for Jace's father, Valentine. They put the screws to Luke at his bookstore, but he always seems to have the intellectual upper hand. He has the physical upper hand in the end… when he kills them both.
So why are they even in the book? Every maniacal bad guy needs some expendable muscle, and that's exactly what Blackborn and Pangwell are. Oops, we mean Blackwell and Pangborn. See? We almost forgot their names. They're like the disposable Dixie cups of bad guys.