Well, from what you and Dave Blackmore have posted concerning James Blackmore (1743) and Rachel Owen, it's obvious James (1743) was not married to Mary Dawson. That leaves James (1764), son of Samuel Blackmore Jr. as the most likely "James" that married Mary Dawson. We know James (1764) would have been in Washington Co. as his father Samuel and family was there by the early 1780's. Samuel died there 1787. Washington Co. borders Beaver Co., where there were a lot of Blackmore's and Dawson's. So yes, I'm saying James (1764), son of Samuel Jr. married Mary Dawson, but I can't prove it.