The phrase "black sequins" refers to small, shiny, flat discs that are black in color and often sewn onto clothing or accessories for decoration. Full definition
A little black sequin dress, but this time, matte shine. (wendyslookbook.com)
I wore this one with a sweatshirt somewhere else on the blog... and there's a long black sequin skirt with a chunky cardigan somewhere (you've already go that!). (thesequinist.com)
This look is so similar, but with a purple skirt and black sequin jacket (not in the picture). (magnoliasandsunlight.com)