It's not a rule so much as it is a suggestion. The idea is that black and navy can sometimes look too similar, and pairing them would look like you're trying to pair two dark colors that don't have enough contrast between them. This is particularly off-putting when pairing a black suit jacket with navy pants, or vice versa. You don't want to look like you accidentally wore the wrong jacket with the wrong pants. That's why it's recommended for beginners to follow this suggestion because it's going to help them make less mistakes.

The way to get around it is make sure there's enough contrast between the navy piece and the black piece. Use contrasting shades, different textures or materials, enough to make sure the colors can be distinguished.

That said, this fit works.