There is a common misconception that wearing all black is universally flattering and slimming. This is a lie.
While dark colors do often hide imperfections and give the illusion of being thinner, assuming that the more black you wear, the more slim you look is incorrect.
Take exhibit A: the outfit below. When I first tried it on, I had a black top underneath the blazer; I thought the pink was enough color for this particular day. However, I am short waisted, so I don’t look very good if things hit me too high, like the jacket does. Although the black shirt was longer than the jacket, because it blended in with the pants, the overall impression was that I was cut off at the end of the jacket. I looked squished, frumpy, and therefore heavier.
By switching out black for a color that stood out – in this case, green – I was able to extend my torso visually, which overall made me appear taller and therefore thinner. With this outfit on me, black was very much not more flattering or slimming.
(BTW, the pinstripe in these pants is actually red, but against the hot pink, you can’t tell. That’s a trick I often use to expand my mix and match options. The smaller the print, the more subtle the color and the more easily it can be swayed by whatever is against it.)