The update went through like before. After logging in I had the black screen again, however this time I kept the PC running for 8 hours. In the morning, at least I was able to see the task manager. Running explorer or "cmd" had no immediate impact though, it just seems that everything is running very slowly if at all. I found a similar problem on HP's forum - users having black-screen problems in combination with the AppReadiness service due to faulty registry entries (by some HP driver). I do have an HP printer, maybe this is part of the problem. Will try disabling the AppReadiness service, provided I can get access to services.msc within a reasonable time. Is there something else that might help?