Several good metal toning fluids are available from Bluejacket (www.bluejacketinc.com). I don't know what problems may come up in shipping such chemicals overseas, though.
Another chemical that will work (according to reliable sources; I haven't tried it myself) is gun bluing. In the U.S. it's available from virtually any store that sells rifles, down to and including Wal-Mart. I don't know how widely-available such things are in Europe.
In any case, it's always a good idea to precede the blackening process by rinsing the brass in white vinegar, and to follow it with a quick spray of flat lacquer (e.g., Testor's Dullcoat). Otherwise the blackening tends to rub off.
Good luck.
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