How quickly are those bubbles forming? If they are forming visibly quickly then there is an acid/metal reaction forming hydrogen gas. However if they are forming over an hour or more then it's just disolved gases coming out of solution. Same thing will happen to anything, even glass, no reaction taking place. Distilled water won't give you the bubbles in this case because all the gas has already been taken out.
The fact that it turns black however is worying. The only time I've seen that happen was when I dipped some copper in a volcanic spring.
Edward
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