I didn't get black ornaments until I hung a plain glass tree one year and it looked like nothing, just a sparkly blur from across the room. The next year I threw in a dozen matte black ones, no pattern, no plan, and suddenly the whole tree had a shape to it. Lights catch on something instead of just bouncing around. That was the moment it clicked, standing in my living room at eleven at night wondering why a cheap box of ornaments made more difference than the expensive string lights I'd just bought.
Where they go wrong is anywhere already busy. A tree loaded with tinsel, plaid ribbon and six different metallics does not need black thrown in too, it just reads muddy, like the tree needs glasses. Black works best as the quiet one in the room, the thing that gives everything else somewhere to land. It also clashes a bit with anything very rustic, lots of burlap and pinecones, because then it stops looking intentional and starts looking like you ran out of the right ornaments and grabbed whatever was left in the bin.
Finish matters more than I expected. Glossy black is handsome but shows every fingerprint and every bit of dust, which in my apartment means it shows a lot. Matte black is the forgiving one, it absorbs light instead of throwing it back at you, and it hides handling marks so well I genuinely cannot tell which ones Marcus has grabbed by accident off the shelf. There's also a soft, almost chalky black finish that feels nice in the hand, lightweight, a little rough, and it makes a satisfying dull clunk rather than a cheap plastic clack when it knocks against another ornament. If a box feels too light and too shiny, it's usually hollow plastic and it will crack the first time it rolls off the couch.
If you own none of this, start with a plain round shape in matte black, nothing shaped like a star or an animal yet, just the basic ball in two or three sizes. That gives you something to build on every year without committing to a theme you'll regret by January. Glass ones feel heavier and sit better on the branch, they don't bounce around in a draft the way the thin plastic ones do, though they obviously need a gentler hand coming down from the closet shelf.
The one mistake, don't buy an entire box in a single size. A tree full of identical black balls looks like a sample display, not a tree. Mix at least two sizes in the same box or buy two separate boxes, your eye needs the variation or the whole thing goes flat.