In its FireRed Pokédex entry, it is stated Poliwag has black skin. The Ruby and Sapphire Pokédex entry for Poliwhirl states that Poliwhirl's skin is covered in oily fluid - if the same applies to Poliwag, it might be a case similar to the common bilberry: the berry itself is black but covered in a wax-like coating that makes it look blue.
/thread
