I just discovered that the slideshow functionality in Lightroom 5.5 on my Mac yields only a black screen.
This is the case for both impromptu slideshows (CMD-Return) and the Slideshow module.
From a Google search and looking over topics in this forum, I've found other posts describing this identical problem for both Mac and Windows versions of Lightroom going back to the original introduction of Lightroom 5 over a year ago.
I've been using Lightroom on various Mac computers since the first Lightroom beta release over 10 years ago. I generally don't use the slideshow functionality of the program, so I was quite surprised to discover yesterday that it's not working in Lightroom 5.5. I originally tried to preview/play a slideshow with 100 photos--some Olympus Raw files and some Canon JPEG files. I have since replicated this problem with a test collection of 5 JPEG images.
I know that the slideshow functionality used to work back in Lightroom 3 and 4. I hadn't tried to use it in Lightroom 5.x until now, so I don't know when it became broken, but it's definitely broken now...and from reading various posts here on Adobe forums and in related non-Adobe forums, it appears that Adobe has yet to acknowledge that this problem exists...which I find surprising and disappointing.
The only work around seems to be to export a PDF slideshow from the Lightroom 5.5 Slideshow module, which does produce a full-screen slideshow PDF. But, the slideshow functionality should work directly in Lightroom, as well.
Hardware: Mac Book Pro Retina -- fully loaded (2.6 GHz Intel Core I7, 16 GB RAM, Nvidia GeForce GT 750M 2048 MB graphics card, External Apple 27" Cinema Monitor connected via Thunderbolt)
OS: Mac OS X 10.9.4 Mavericks
LR Version: Lightroom Mac 5.5
Problem 1: Impromptu Slideshow (pressing CMD-Enter/Return keys from Library/Develop modules) causes:
1. spinning beachball for seconds to minutes depending on how many images are in current folder/collection
2. followed by "Preparing Preview progress bar
3. followed by entirely black screen. Pressing forwards/backwards arrows do nothing; pressing ESC or mouse click returns to Lightroom module
Problem 2: Slideshow Module - Clicking Preview button (or Preview triangle under slideshow window) causes:
1. Preparing Preview progress bar
2. followed by slideshow window turning completely black.
Clicking the Play Slideshow button exhibits exactly the same behavior as above, except that my entire screen blacks out instead of just Lightroom's slideshow window.
Just upgraded from Lightroom Mac 5.5 to 5.6.
Per my post above, creating an impromptu slideshow (CMD-Return) or using the Slideshow module produces either a completely black preview or a completely black full-screen slideshow.
While Adobe addressed other issues and bugs in Lightroom 5.x, they apparently did not address the slideshow functionality in this release, alas....