This roux should almost be twice as dark as the light brown roux but not as dark as chocolate. You should remember that the darker the roux gets, the less thickening power it holds and the roux tends to become bitter. This roux is used most often in sauce piquantes, crawfish bisques and gumbos. However, it is perfectly normal to use the dark brown roux in any dish in Cajun cooking. This roux gives food such a rich character that I sometimes make shrimp and corn bisque with it, as well as a river road seafood gumbo that will knock your socks off.
Slow cooking is essential to achieve that dark, rich color.
Some time ago, I was discussing the origin of the dark roux with my good friend, Angus McIntosh, a chef and aspiring Cajun. I've always contended that because the Cajuns cooked in black iron pots over open fires using lard as a base, the dark roux was discovered by accident when the fire got too hot and the flour over-browned. With their lean pantries in mind, the Cajuns kept the roux instead of discarding it. They enjoyed the flavor and kept doing it that way. Thus, the origin of the name.