They aren’t officially announced yet, but that announcement is likely coming next week. If you pre-order from Best Buy, the delivery date shows as May 15th. Previous movies that displayed the delivery date on a pre-order, such as The Last Jedi, did actually release on the date displayed. So that means we’re probably going to see the Black Panther on May 15th, with the digital release traditionally being two weeks earlier.
Here are the 4K and Blu-Ray covers: