“WASHINGTON — The 200-yard black cloth stretched along the foul lines, starting at one edge of the outfield, wrapping around home plate and extending to the other side of the diamond. It was held by coaches and players from both the Washington Nationals and the Yankees, all spaced out, as a message recorded by the actor Morgan Freeman played over the stadium speakers.

Then they all took a knee for 60 seconds of silence — an idea agreed upon by the Yankees in a team meeting the night before Thursday’s season opener and then shared with the Nationals. For the national anthem, both sides stood again.

This was not the N.B.A., or W.N.B.A. or N.F.L., where players have been demonstrating before and during the national anthem for years. But what happened on opening night was notable for Major League Baseball, a league that has been slow to address social issues publicly compared to many of their counterparts.

Before the recorded message and the kneeling, a Black Lives Matter video produced by the Players Alliance — a new nonprofit comprising 150 current and former Black baseball players — was played on the stadium’s screen. The video, originally released last month, featured the Yankees stars Giancarlo Stanton, Aaron Judge and Aaron Hicks, among others.