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...