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I'm still using my slate 5 while I wait for the black 7 to arrive, but I compared them in a shop the other day and to my eye they are quite different. The 7 is very pure black, where the 5 has a greenish tint in some light.
 
The Slate iphone 5 had a very slight blue tint to it if you looked at the edges in bright light, which is why I assumed they chose not to just call it black (other than marketing I guess). By Apple choosing to call the new one "Black" instead of "Slate", it's reasonable to assume that they are in fact different colors.
 
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