Separating one face into two with an edge I am looking to separate the body of the sword into two faces, so that I can easily make the top half black and the bottom silver. 

Simply adding an edge in the middle did not separate the blade into two. 

I created the edge by directly connecting the vertex at the tip with the vertex near the handle, shown in the picture. 

Any tips appreciated!
I'm guessing you created the edge with F? Try using the Connect Path tool instead (J):