I am adding on to Kara the Computer Tutor's message. In addition to unchecking "border lines," in my experience, it was essential to also uncheck "bottom" in order to remove the black line.

Hi,
this line is nothing but the style applied. try clearing style format and it will be gone

You have resurrected a thread that is more than ten years old.

That said...Everything in Word has a style applied.

Lines like this are borders, usually direct formatting, not style-based. The solutions given above address any methods that can cause such a line.

Yes, reapplying a style will remove direct formatting.