Assuming Netbeans 7.0 +

Select Tools from the Top menu bar

-> Options

-> click on Editor ( looks like a book )

-> select the tab labeled "hints"

-> from the pull down menu choose the correct language

-> deselect the hint for that feature

If Thats a documentation prompt then

-> select the tab labeled "Code Completion"

->deselect "Auto Popup Documentation Window"

In my Neatbeans however HINTS or DOCs are never on black background so i can't rule out that you have a addon or plugin that may be causing this ...