I had a section of trim that I copied using right click, copy to symmetric pattern.  I later removed symmetry, to work on something particular to the left side of the garment.  I didn't notice at first, but the trim that had been pasted ended up with flipped normals, and showed black in the simulation window.  It definitely wasn't a visible issue when  they were first pasted, so I assume it happened when I removed linked editing.  There also doesn't seem to be any way of flipping normals on trim, so I deleted and added them again individually.