Why is that hard to imagine? Especially since Bush (also as a sitting president) did exactly that. And Romney and McCain have both been invited to speak to the NAACP, with the former accepting the invitation. Trump getting an invitation doesn't seem surreal at all.
Yes, I too knew they had done just as you indicate. What is surreal is the energy and response of this event compared to the ones you mention. This was not an obligatory type appearance on either party like the Bush NAACP speeches. These pastors invited and wanted their followers to hear the President, some of them are open supporters of his reelection campaign.
The press knew it was different, instead of laughing off an ineffectual overture to the black community the press either ignored or pounced on the pastors, calling it a campaign rally. I wish I could post a link that would show the difference, but the best I can do is tell you that I have been a follower of politics for years and this just has a different feel to me, how that translates is anyone's guess.
I believe we may be on the cusp of a very logical but earthshaking political realignment. The democrat party long ago left the values of the black community, particularly the church supporting. Evidenced by the California proposition on gay marriage, it was defeated lead by the black community voting 60-80% against, democrats overturned the will of the people. The republican elite left the values of the so called evangelicals, flyover country, small business, and blue collar workers. Think Paul Ryan and Koch brothers with their free trade with communist governments looking to hurt the US and uncurbed legal and illegal immigration.
I think the message to the black community is WELCOME TO THE NEW REPUBLICAN PARTY!