All protocol, except Libre, print in black. Libre documents print in light gray
And what is the question?
Read these guidelines and edit* your description to improve it. Mention your OS name and LO version.
Describe what you do and in what it is different from what you expect.
You tagged draw, meaning you are using Draw, the drawing application. The default colour for lines may be set to light gray or you set this colour through some drop-down menu.
@Kobra, When you choose menu File - Print…,¹ the preview is in black or in gray? Try with a large shape, not only borders or small text.
¹ See also the Color options in the LibreOffice Draw tab.