Homebrew PCBs

Gun Blueing from your local hardware store.
Very slightly dampen a corner of a cloth with it, rub briskly.
You'll get a black sooty deposit if you put too much on / leave
it wet etc.; start with a less dampened corner of the cloth.
Keep rubbing....

Same deal as prep for etching / plating - clean.  Kitchen cleanser
like ajax etc.  Wash well.

Rubber gloves - it leaves a nasty odor that's hard to wash off.
Nothing terrible in the MSDS.

CHeaper than most of the brass-blackening compounds in the few
hobby shops that sell them.  Faster, too.
/mark