dirt and moisture between two keys. also, never wipe towards backward, this method is wrong because it pushes dust or dirt in the space between two keys or behind the keys. do not wipe black keys with the same cloth you used for white piano keys.
take another new white cotton cloth to wipe or clean black keys, carefully clean black piano keys by using the same procedure of soapy solution. you can clean black piano keys from front to backward, but make sure it does not touch the cleaned white keys. if you touch the cloth to white keys it may transfer black color or dust to your white piano 