I have no knowledge of the origins of the name Blackmore as a middle name for William. I originally asumed it was a family name.
Recently I was reading a book "Early Times in Middle Tennessee" published in 1857. The author, John Carr, mentions a Maj. George D. Blackmore and sons Dr. James Blackmore, " who lived in sight of Gallatin" and Gen. William Blackmore. clerk of the chancery court who died in the 1830's. Is this the origin of the name? The youngest son was named Zachariah Taylor Williams.