When silver is exposed to air it becomes black due to the formation of a layer of oxide coating.....though silver is not a very reactive with the passage of time it slowly reacts with the oxygen in the air and forms black coloured silver oxide......

*Silver is  not the only metal to do so...even gold articles become black 
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Onely silver ..wat abt copper dude!!! mistake,,
Silver articles become black because they are gets coated with atmospheric Sulphur gas .
They are also called Self Protecting metal as the coating helps them to acquire waterproof and Moist proof coating around it ..
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Just with ...sulphur !!
oops my mistake...