GRAINS-Wheat slips on easing concerns over Black Sea supplies

* U.S. wheat drops after 2 sessions of strong gains * Easing concerns over Black Sea wheat supplies weigh * U.S. rains reduce concerns about scattered dryness (Updates with U.S. trading, previous dateline SINGAPORE/LONDON) By Tom Polansek CHICAGO, Aug 18 (Reuters) – U.S. wheat futures slid 1.5 percent on Monday, giving back some of the