A dark roux is the foundation of Cajun cooking. It is a simple mixture of equal parts flour and fat, cooked over medium heat for 35-45 minutes until it reaches the color of dark chocolate. In that time, something remarkable happens: the flour undergoes a Maillard reaction that transforms it from raw starch into a complex, deeply nutty flavoring agent.
The roux contributes three things to a gumbo or sauce: color, flavor, and thickening power. As the roux darkens, it gains flavor but loses thickening power. A blonde roux is an excellent thickener but has little flavor. A brick-red or chocolate roux has enormous flavor but relatively little thickening power -- which is why a good gumbo also uses okra or file powder.
The One Rule You Cannot Break
You must stir constantly. A roux left unattended for even 30 seconds at the wrong heat will burn, and a burnt roux cannot be saved. It must be discarded and started over. Do not answer the phone. Do not leave the kitchen. Give the roux your full attention for the entire cooking time.
Heat and Color Stages
Start over medium heat. As the roux progresses from white to blonde to peanut butter to brick red to dark chocolate, lower the heat slightly. The darker the roux, the easier it is to burn. Most experienced Cajun cooks lower from medium to medium-low around the peanut butter stage and to low at the brick-red stage.
The classic "chocolate" roux for gumbo should be the color of unsweetened dark chocolate -- not bittersweet, not milk chocolate. If you reach that color without burning it, you have succeeded.
Aromatics and the "Holy Trinity"
When the roux reaches the right color, add your diced onion, celery, and bell pepper immediately. The vegetables will sizzle violently and drop the temperature of the roux, effectively stopping the cooking. Stir constantly for another 8 minutes as the vegetables release their moisture and the roux loosens and becomes more fluid. Add garlic last.