Three ways to build the outfit
The easiest entry point is what I think of as the big block and the small punctuation. Wear one colour as the main event — a navy trouser and knit, say — and let black arrive only in the accessories: the boot, the bag, the belt. It reads as intentional immediately, because nobody accessorises by accident.
The second approach is texture-led. Put a matte black wool coat over a navy silk shirt, or a chunky navy cable knit over slim black leather-look trousers. When the surfaces differ dramatically, the colours stop being compared to each other and start being appreciated separately. This is the version that looks most expensive for the least money, because texture is doing the work.
The third is the split at the waist: navy on top, black on the bottom, or the reverse. Keep the line clean — a tucked shirt, a defined waistband — so the meeting point looks like a design choice. If you feel exposed, a slim belt in either colour, or in a warm tan, settles the whole thing.
The details that stop it looking like an error
Check your navy in daylight before you commit; artificial light flattens the distinction and you may find your navy is actually reading as very dark grey. If a piece is faded or bobbled, retire it from this particular pairing and keep it for weekends.
