Very interesting! Thanks for all your perspectives.
A little more info; the top is a a white top with tiny black polka dots; shortsleeved, voluminous, hipgrazing, button front with small black fabric covered buttons, no collar, v-neck (not low), with black lace trim around the neckline. I think this is partly why I liked the black under it. (The tank was a rust red that blended in with my skirt, which was a muted red with small cream polka dots). The skirt is vintage, probably nineties, knee-length, fit-and-flare with a high waist and interesting waistline (a v-cut in the centre at the back of the waistband). I did not tuck the top.
I have medium skin (my Dad is Indian. My body would be about Mac shade NC40) so black doesn't blend, but if it did the look would not have caught my attention.
My breasts are heading back to their pre-children A cup if that's relevant.
It ended up not being an issue as I started the day with a tank top underneath and was far too busy volunteering at the fair to be in a space to feel more relaxed and pretty. I had zero energy afterwards so did not take a photo. Right before I left I did slip off the tank and felt out of my comfort zone (though relievedly cooler)!
Both looks were problematic. With the tank the outfit was meh. I looked like I had no boobs my waistline was obscured a bit even though the tank was fitted. Also, the special waistband was not visible as it blended in with the top.
With the bra, I felt conscious that I was showing my bra (surprise, surprise). It was frustrating because the look *almost* worked. With the high waist of the skirt only a few inches of belly were visible - probably the part of my body most worthy of accentuating. If I was wearing a sari, I'd be showing the same thing and it'd be modest. You could also see how nice my skirt was while veiling it through the blouse which I liked.
I think this would have worked with something that didn't signal "bra". I don't think bras look bad but it is more the ideas people have in their head about them. Something that has a straight edge at the bottom instead of the roundness of "bra" cups. That might also give even an inch or 2 more coverage; even though my ribcage isn't obscene, covering it slightly may shift the association. I might keep my eyes open for a... wait for it... bustiere! (Now how's that for nineties retro)