Rikk,
Good idea -- I hadn't tried trashing my Lightroom preference files, so I just now did that....
But, alas, it didn't help.
I had Lightroom preference files going back to the original version of Lightroom, so, after closing the Lightroom application, I dumped all my Lightroom preference files into a folder on my desktop. Then, I restarted my Mac to clear the system cache, and then I opened Lightroom 5.7. It was clearly using a new preference file because it opened full screen, whereas I had resized the Lightroom window to about 2/3 of my screen width.
I then selected a small test collection of 5 photos and clicked on the Slideshow tab. As happened when I upgraded to Lightroom 5.7 a few days ago, it took 3-4 minutes for the Slideshow module to initialize (spinning beachball). But, I eventually regained control of the interface.
I had a simple layout set up with a gray background and a 2 px border around the photos. I could click the arrow buttons at the base of the main window and cycle through the photos. I made sure that all 5 images in the collection were selected for the slideshow and clicked the Preview button...and the main Lightroom window went black. I then clicked the Play button to play the slideshow full screen, and again, my whole screen went black. During playback, pressing the arrow keys on my keyboard does nothing; likewise the space bar. If I click anywhere on the screen or press the ESC key, the [black] slideshow closes and the Lightroom interface returns.
So, apparently there's something else going on here....
By the way, just for the heck of it, I fired up my old MacBook Pro, which was a 2011 vintage computer on which I have both Lightroom 4.x and Lightroom 5.2 installed. I first opened Lightroom 5.2 on that computer, imported a few photos from my Pictures folder and tried the Slideshow module. Same black screen problem on that computer--black preview and black full screen playback.
Then, I closed Lightroom 5 and opened Lightroom 4.x (I can't remember what version, but it was the last LR 4 available). Once again I imported a few photos from my Pictures folder and then tried the Slideshow module. This time, it worked just fine. The preview played properly in the main Lightroom window, and the full-screen playback worked just fine.
Any other thoughts or suggestions???
BK