By going though the updates, one by one I found the culprits: 
Packages Altered:
    Upgraded libkworkspace5-5.8.5-5.fc25.x86_64                    @updates
    Upgrade                 5.8.6-2.fc25.x86_64                    @updates-testing
    Upgraded plasma-lookandfeel-fedora-5.8.5-5.fc25.noarch         @updates
    Upgrade                            5.8.6-2.fc25.noarch         @updates-testing
    Upgraded plasma-workspace-5.8.5-5.fc25.x86_64                  @updates
    Upgrade                   5.8.6-2.fc25.x86_64                  @updates-testing
    Upgraded plasma-workspace-common-5.8.5-5.fc25.x86_64           @updates
    Upgrade                          5.8.6-2.fc25.x86_64           @updates-testing
    Upgraded plasma-workspace-drkonqi-5.8.5-5.fc25.x86_64          @updates
    Upgrade                           5.8.6-2.fc25.x86_64          @updates-testing
    Upgraded plasma-workspace-geolocation-5.8.5-5.fc25.x86_64      @updates
    Upgrade                               5.8.6-2.fc25.x86_64      @updates-testing
    Upgraded plasma-workspace-geolocation-libs-5.8.5-5.fc25.x86_64 @updates
    Upgrade                                    5.8.6-2.fc25.x86_64 @updates-testing
    Upgraded plasma-workspace-libs-5.8.5-5.fc25.x86_64             @updates
    Upgrade                        5.8.6-2.fc25.x86_64             @updates-testing
    Upgraded sddm-breeze-5.8.5-5.fc25.noarch                       @updates
    Upgrade              5.8.6-2.fc25.noarch                       @updates-testing

I didn't narrow it down any further because of package dependencies, but if you dnf downgrade the above packages, you'll eliminate the black screen issue.
I'll create a bug...

On Sat, Feb 25, 2017 at 6:54 AM, Ed Greshko <ed.greshko@greshko.com> wrote:
On 02/25/17 22:37, Gerald B. Cox wrote:
>
> On Fri, Feb 24, 2017 at 9:28 AM, Rex Dieter <rdieter@math.unl.edu
> <mailto:rdieter@math.unl.edu>> wrote:
>
>     FYI, sent of kf5-5.31.0 and plasma-5.8.6 to -testing (both
>     kde-testing and
>     updates-testing) repos yesterday ,
>
>
>
> Hey Rex,
> Immediately after the upgrade started receiving the "black screen"
> issue others have reported.  :-(  Sending this note from lxqt.  kde
> for me right now is unusable.
>

FWIW, on an older system which was behind on updates I updated
everything simultaneously.  There was a kernel update and others thrown
into the mix.  After a reboot I had a "black screen" and a bunch of
errors in my  Xorg.0.log that I failed to save.  :-(

Anyway I disabled the kde-testing and updates-testing repos and did a
"dnf distro-sync".  This gave me a usable system again.

I then enabled kde-testing and updated and rebooted.  Now everything
still works.

Not sure why....


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