Ribier grapes are  large sized grapes that have seeds, and have firm, slightly bitter skin, and meaty flesh, and  mild sweet flavor. When ripe,  Ribier is a dark black/purple color. The bunches should be medium length, squaty and full of large round grapes. 

Ribier grapes originated in France. It was imported to South Africa in 1926 from Argentina.  It is also known as Ribier in Spanish speaking countries and as Gros Noir (large black) in France. In Chile,  Alphonse is an early black cultivar and is picked from the last week of January until mid-March. 
The berries are large, round and a deep black colour.  Very sweet and juicy. 

As with all grapes, Ribiers are low in sodium, high in anti-oxidants, have food amounts of fiber, and are low in calories, with 70 per cup. Grapes do not get sweeter after picking, so color is the best indicator for ripeness and sweet flavor. Look for grapes that are plump and firmly attached to pliable stems. Avoid soft or wrinkled fruits.

Grapes are sold ripe and ready to eat. Refrigerate unwashed. They will keep will for up to 7 days at 38-40 degrees F. Just before service, rinse grapes with cool water. 

One pound of Ribier grapes with yield 2 1/4 cups fruit.

Here are three in-depth and excellent grape sites for information, recipes and more:

http://www.tablegrape.com/rev99/index.htm

http://www.dole5aday.com/encyclopedia/Grapes/grapes_types.html