There are many names for the dark roast that is more black than brown, has rich and copious levels of surface oil and is roasted to produce deep carbony, smoky, flavor notes with perhaps only 10% of the actual coffee flavors remaining. You see this dark roast called Italian, Italian Espresso, and a host of other proprietary names like Rocketman Blend, Foglifter, etc.
A roaster can make a French Roast out of one single origin coffee or from a blend of many coffees of different origins. What you need to know is that 90% of the flavor is derived from carbonization of the coffee beans, not from the careful cultivation of the coffee tree by a caring farmer.
Note: be careful with this dark colored coffee because often it is a dumping ground for mediocre and less expensive beans whose flaws are easily masked by burning them out at the high heat needed to make a French Roast.
What to look for: large, uniform, shiny beans. Single origin French Roasts are rare, but if you see a bin card that defines its French Roast as from a single origin, one country, select it before you select a French Roast blend. Chances are the roaster is proud of that coffee and your selection has a better chance of being of pedigree stock.
French Roast is a strange name for the darkest of roasts. Do the French drink coffee this way? I doubt it. I believe early artisan roasters during the 1960s created this designation, believing erroneously that this carbonized vegetable matter was a European approach to coffee. The beans do not originate in France.
A well-made French Roast should have burnt sugar notes, licorice and roasted chestnut flavors and a long wet (not ashy) finish.