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Thuja occidentalis (arborvitae) is a very resistant tree to water, air pollution, diseases, insects. Your local nurseryman is not too far, but I'm skeptical if too much water is the problem. Water excess in the soil won't bother too much during few days, few weeks. But water excess in the air and warm temperature, help a lot diseases to develop. It doesn't look exactly the symptoms when it's water excess in the ground. Arbovitae is resistant, but not "untouchable".
You said foliage turned black, brown and died.
Did you notice other symptoms, signs?
When the foliage is black, do you feel a rough sensation with your fingers when you touch it or does it look inside the scales of the foliage?
Are there other colors like orange, yellow?
Does it look like spots or is it completely colored (black, brown)?
Are there spots like that on the branches?
I have some clues, but we have to go further before to try something!
Andre R.