Evergreen trees are often affected by "sooty mold." This mold is dark blue or black in appearance and it grows on the honeydew excreted by insects preying on your tree. Sooty mold, in most cases, causes no harm to your tree. However, if it covers most of your evergreen's foliage, it may interfere with its ability to photosynthesize. Sooty mold is quite easy to get rid of. However, you must control the insects at the root of the problem or it will simply come back as they produce more honeydew.
Add 1 tsp. of liquid dishwashing detergent to 1 gallon of room temperature water.
Dip the rag in the solution and wipe the sooty mold off the evergreen's foliage. While you are getting rid of the mold, look around for the insects that are excreting the honeydew. Check in crevices and crooks in the tree where they might be hiding.
Spray the tree's foliage with a hose to rinse away any residual detergent.
Collect any insects that you find and take them to your county extension office or gardening center to help you identify them. Aphids, mealybugs, scale insects, leafhoppers, psyllids and whiteflies are common culprits (you may be able to identify them with an image search). After you correctly identify the insect, purchase the appropriate pesticide and treat the tree according to the manufacturer's instructions.