Dun (Dn is the genetic abbreviation) is a dominant dilute (modifier) gene.  Dominant means the characteristics show up if the dun gene is present, and that it can dilute/modify the base color.  The nomenclature has changed recently for dun on a black base, formerly called grulla, now is called black dun. (Fig 1 - Handsome Dun Roadster) There is a defined stripe down the center of the horses' back, horizontal bars on the legs, striping horizontally on the ears plus a perpendicular (to the dorsal) stripe across the withers. 

This is Champagne Dun Darlin', a classic dun champagne TWH filly. (Fig. 2 - Champagne Dun Darlin') For champagne, classic means a black base color, so she is Dun + Black + Champagne.  Here you can see the dorsal stripe, the shoulder barring as well as barring on her neck. Note the dark rims on her ears.

Some horses have what is called "counter-shading," (Fig. 4 - Champagne Shadowfax)a darker area down their backbone, but not cut-and-dried distinct line like a dun has. (Fig. 3 - Champagne Dun Darlin')

vs.               counter-shading

Above is hind leg barring, (Fig. 5) and to the right is front leg barring on the classic dun filly.(Fig. 6)

Ear barring shoulder barring on Handsome Dun Roadster, my black dun colt. The darker face is common with black duns.  Mane, legs and tail are black with lighter body like a bay.(Fig 6 - Handsome Dun Roadster)