This year’s box office so far has been a story of one completely dominant movie, “Black Panther,” highlighting a potentially troubling trend for Hollywood in which ticket sales are increasingly concentrated among just a few ultrasuccessful pictures.

With $650.7 million and counting, “Black Panther” is on track to become the third highest grossing movie ever in the U.S. and Canada. It accounted for 23% of all ticket sales in the first three months of the year, ending Saturday, according to comScore. That is the second-highest...