When KDE Desktop shows a black screen after distribution update. Well, it displays the load intro animation, then black screen. The safest method is to turn “compositing” features off. Get a prompt (ALT+CRTL+F1), login in with your user name / password and enter:
kwriteconfig –file kwinrc –group Compositing –key Enabled false
You could also manually edit the kwinrc file in .kde.