Perhaps not common, there were highly skilled black tradesman in the 18th and 19th centuries - one example would be at George Washington's Mt. Vernon estate. At it's peak the estate had 316 slaves, many of whom were trained "in crafts such as milling, coopering, blacksmithing, carpentry,and shoemaking."
http://www.mountvernon.org/learn/meet_george/index.cfm/ss/101/
In 1792 GW had a sixteen sided timberframe threshing barn built on the estate and it would be reasonable to assume that they would have worked on that along with many of his other structures.
http://www.mountvernon.org/learn/explore_mv/index.cfm/sss/76/
All of Washingtons slaves were freed on his death. I'd guess odds are good they probably didn't work directly on the Portland church, but given the itinerant ways of the timberframer (then and now) the timing is such that they may have helped form part of the labor pool of people who mentored or apprenticed the people who did.