First I tried everything with a window, but i got the black screen so started commenting out lines of code. Even when I have nothing besides the Create() function the screen stays black. I dont even try to render.
If you don't render anything, it is expected that the screen stays black. When you call sf::Window::Clear() it clears the screen to the background color (Black by default). Merely Create()ing a button or any widget for that matter will not result in you seeing it on screen. You have to render it to the screen to see it.
SFML renders everything in
immediate mode meaning that if you don't explicitly render an object you have created every frame you won't see it. Because SFGUI builds on SFML it too renders everything in
immediate mode meaning that you have to call Expose() yourself every frame to render any widget to the screen.
You may think that SFGUI retains a list of Widgets that you created and renders them automatically every frame, but this is not the case.
If you don't render anything yourself, you also won't see anything.