When I turn on my Samsung Galaxy S4, it often comes with a black clock/notification screen.
How to turn it off?
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Sounds like a lockscreen widget, anyway try this:
Drag down the screen from where the message and clock is. Delete text and uncheck clock then press save (just needs to be edited from the lock screen).
You may need to disable your lock security while editing. Then tap save in the top right and your lock screen will be clear