How can one darken brass?

[Rich]

Try Brown's Patina or Baldwin's Patina available from
Reactive Metals.

http://www.reactivemetals.com

I've dealt with them. Ask for their catalog; everything
imaginable in the way of metals and metal working
materials and patinating solutions. All items are
not on their website, so ask for a catalog or email
them about the above agents.

Rich S