As a source of neutral midtones to set color balance from, I mean.
Black leather jackets. Asphalt pavement. Black t-shirts. All of these, if they have highlights going on, can easily get tones up into the midrange (which I think of as 90-180, roughly) suitable for use with the gray eyedropper for setting color balance.
If it’s dark mixed with specular highlights, though, it’s usually not useful; specular highlights are by definition not a useful source of color balance, since the ability of the sensor to capture the light falling on it has been exceeded. The ratio of colors in a specular highlight pixel doesn’t tell you anything about the actual color of the light falling on it there.