A velvet dress in black is the easiest formal choice for a Christmas party. Black velvet looks expensive even when it is not. The nap of the velvet gives black depth that regular black fabric cannot match. The color pairs with literally any accessories and any shoes. Gold jewelry or silver jewelry. Red shoes or black shoes or metallic shoes. The simplicity of black means the construction of the dress matters more. A well-cut A-line dress, a fitted sheath dress, a wrap style. The silhouette carries the outfit when the color does not. Black velvet works at any formality level depending on how you style it. Casual shoes and minimal jewelry reads different than heels and statement jewelry. The fabric needs to be substantial enough that it drapes properly without clinging to every lump and bump. Stretch velvet is more forgiving than rigid velvet. These are the velvet dresses that work because black is reliable and velvet adds the texture that keeps it from feeling boring.