Answer (first posted by @patrickBOOTH ): It is going to be very hard to notice any difference in shoes that are already black just using polishes. I have a pair of shoes that I striped down and used leather dye, many coats of navy and a few black coats over it and many would barely notice a difference. In any event, using navy polish on black shoes certaintly wouldn't hurt. Cream deposits more color, but wax shines better.
Follow up answer from @Gdot : I've used Navy on black shoes both creams and polishes. My experience matches Patrick's, it just never adds up to anything more than a faint glow of blue. It's nice - but no one would notice who wasn't looking very closely.
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