I suspect that what you are seeing is a layer of sooty mold on the leaves and needles of your trees.
Sooty mold is a charcoal black fungus that appears as a black coating on the surface of leaves, fruits, twigs, and branches of many deciduous and evergreen shrubs and trees. It is not a fungus that causes disease so it is not directly harmful to the plant. However, heavy coatings of sooty mold may affect the plant indirectly by blocking sunlight and air from reaching the leaf surface and thus it can interfere with the process of photosynthesis.
The growth of sooty mold usually indicates that some type of piercing, sucking insect such as whitefly, scale, mealybugs, spider mites, or aphids has attacked the plant and these guys can do real damage if they are not controlled. When these insects suck the juices from the plant, they excrete a sticky substance called "honeydew" that drips down and coats the leaves and stems of the plant. The sooty mold grows on the leaf surface using the honeydew as food and eventually covers the leaves with a black, sooty coating.
Aphids, scale insects, and mealybugs will attack Hemlock, crape myrtle, and redwood trees. The trick is to control the insects that are secreting the honeydew that allows the sooty mold to grow. Examine your trees for the presence of insects.
Bonide All Seasons Oil will control aphids, scale. spider mites, and mealybugs when used according to the label directions.
Also the systemic products, Bayer Advanced 12 Month Tree & Shrub Protect & Feed II and Bonide Systemic Insect Control, will control most of these insects. Use only according to the label directions.
Here is more information on sooty mold from our website.