Still experimenting (what I do for you guys!): next day. I didn’t see anyone I knew yesterday—quiet day in the office, and I came and went on little cat feet. So today, doing something I have never done before, I wore the exact same thing on consecutive days, except that today I substituted a pair of pale blue jeans for the dark nearly-navy ones. And here’s my finding: I feel perfectly fine.
So this is my tentative but considered conclusion on the subject: there’s nothing technically wrong with wearing black with navy or dark blue, but don’t do it anyway, unless you’re @elijah or someone whose picture gets put in fashion magazines.
(@Judi is right, of course—we’re really so conspicuous only to ourselves. But since it’s all about confidence and how we feel in what we wear, if we don’t feel right we won’t look right.)
Further research is welcome.