Question
I am surprised that this is outside your expertise...given the information about you areas of knowledge on the website, it would seem to be precisely up your alley!  Do you know of anyone who might have more information on this particular situation (black foliage on arborvitae).
-Thanks, Todd

Answer
I deal with more trees than shrubs but I did find a one reference of black foliage on Arborvitae.

"Over-watering can lead to fungus (black sooty coating) appearing on the foliage. In periods of drought your arborvitae should not need to be watered more than once per week"

I would take a sample to a nursery and ask them if they know what it could be. It could be as simple as an aphid infestation and sooty mold growing on the foliage or a disease that infects the foliage.
Might also check and see if you have an Agricultural Extension Service off near you they have Horticulturist that can ID these type problems.