Copper turns black as a result of oxidation. This happens when there is a reaction between oxygen and moisture condensation, water or rain. When the oxygen that it contacting the penny dissolves, the oxidation occurs and leaves the black color.
In marine, urban and industrial environments, copper tends to resist corrosion well. Temperature, moisture and pollution all have an impact on copper corrosion. Initially, the copper starts to dull and get darker but, after several years, the copper takes on a black color. The latest stage of the process results in copper becoming green due to varying concentrations of carbonate, copper sulfate and chloride salts.