You used to be able to get ammonia from hardware shops & chemists, it was used as a domestic cleaner. Things may be different now due to anti-bombmaking measures.
Stand it in my airing cupboard, all the copper pipes in there have gone green There are ammonia based cleaners available and you can buy it from chemical suppliers in 33% solution. B&Q has it as well
I had a car battery fail and all the copper pipes nearby turned black with the hydrogen sulphide, so crack some rotten eggs over it and wash off with pee.
That probably depends if the black is copper oxide or copper sulphide. I've noticed that some colours of car cable insulation can turn bright copper black.
^^ Fry's certainly does - I assembled some heating pipes, cleaned the ends and applied flux, ready to solder up next day. When I came back all the ends had turned green and I had to start all over again!