I make many detail parts from brass rod and/or brass sheet. For years, I bought bottles of A-West Blacken-It for this purpose. At $9.95 for a 4 oz bottle, it was starting to get a bit expensive. Last year, I bought a 32 oz bottle of Brass Darkening Solution from Rockler Woodworking. They sell it in three sizes: $5.49 for 2 oz, $10.99 for 8 oz, and $29.95 for 32 oz.
That is $2.50 per ounce for Blacken-It vs. $0.94 per ounce for the Rockler product.
I think the results are far better than what was achieved with the Blacken-It. The only downside is that it doesn’t darken white metal casting, such as from Ozark Miniatures or Western Scale Models. It works fantastic though on brass and copper parts.
http://www.rockler.com/darkening-solution
Bob