Bark is a very descriptive word. Take a look at the photo above, the brown/black surface of the meat is called ‘bark’. Bark is formed thanks to the Maillard effect among other things. Do not confuse bark with the charred surface you get on burnt meat, there is a night and day difference with both flavor and taste. The bark looks good and tastes great.
How it forms
Bark is formed slowly on the grill when the spices are mixed with meat juice forming a crust on the meat’s surface. Caramelization only happens when sugar is present.
Juices rise to the surface and evaporate as the meat temperature increases. During evaporation meat juices, spices and smoke form a tasty ’goo’ that sticks to the meat surface. The bark is relatively soft, not at all like the dark crust you get when you accidentally forget a steak on the grill for way too long.
The bark makes the meat taste more meat. Bark needs a lot of time to form, and a low temperature, typically around 225° F. You often see optimal bark on a pork butt, it almost resembles a meteorite, a black rock. This is because it’s been smoked at low temperatures for 12-14 hours.
Taste
The flavor of the bark depends very much on the type of meat. Cooking temperature is important, the bark needs low temperature for a long time to develop.
Last, but not least, the spices and smoke add their part. Some spices are water soluble, others fat soluble. This also affects flavor.
Smoke
If you have a lot of good smoke the bark turns black, and if smoke is lacking the bark becomes dark mahogany red. Air flow, and therefore moisture transportation also affects bark formation. The more moisture in the air, the less bark will form. It’s contradictory, since it is advantageous with moist air in a smoker. One has to compromise.