Answer: Weeping Blue Atlas Cedar would need more sun, and also more space. Although slow growing, in time it grows quite large and becomes a magnificent specimen. In your area (zone 6A or the coldest part of zone 6) it would also need a sheltered location with protection from winter winds. So all in all it probably would not be the best choice for you.
As a rule, needled evergreens do best in full sun all day long, and may tolerate a half day sun. So a scant hour or two of afternoon sun is probably not enough to keep them healthy and growing as dense as they should.
You might be able to grow one of the so-called blue hollies, if you prefer an evergreen, or perhaps a larger growing cultivar of Pieris japonica or possibly one of the larger Catawba rhododendrons. These would provide height and an evergreen presence. However, based on your description I am not certain there is enough direct sun there for any of these to thrive. For this reason you might want to consider deciduous plants that have an attractive branch structure and interesting bark for all season interest.
There are several fctors to take into account in selecting the plant to use. Your choice will depend on the time it is sunny (hot early afternoon sun or less intense late afternoon sun), wind exposure, the soil type, the typical soil moisture levels, and the amount of space available for the plant to grow and mature.
I would suggest you work with your local county extension and/or your local professionally trained nursery staff to analyze the growing conditions where you want to plant and based on that, identify some plants that would thrive in that location. Then, select the one you like best in terms of meeting your design goals. Enjoy your rock garden!
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