The two plain black triangles at the top corners of this mirror are doing more work than anything gilded on it.
An arch inside a square leaves those spandrels. The instinct - and it is a strong one - is to fill them: a scroll, a rosette, a spray of leaves, anything rather than empty space. Almost every maker does it.
But an arch is only read as an arch because of what sits beside it. Fill the spandrels with ornament and the curve loses its edge against them; the eye reads a busy top corner and the shape of the arch dissolves into the decoration. The one thing that makes the curve legible is a quiet area on either side of it.
So they are left plain, and finished in flat black - the darkest, least reflective surface on the piece - so that the gilded arch moulding is seen against nothing at all.
It is a decision that shows only by its absence. Nobody looks at this mirror and admires the spandrels. They see a clean arch, which is the entire point.