Although I do agree with you now that the William Bush of Mongomery Co, GA who died 1808? was not a half brother of Gen David Blackshear I did find this in the book "Georgia Landmarks, Memorials and Legends" by Lucian Lamar Knight on page 720 this paragrapgh which concerned early settlers of Laurens Co, GA:
"William Bush was a half-brother of General Blackshear. He accompanied the General when the latter emigrated from North Carolina to Georgia. The father of General Blackshear married the widow Bush."
On page 528 is the following concerning early settlers to Early Co, GA:
"James Bush, a soldier in the Seminole wars, was an early settler. He was thrice married and reared a family of twenty children. His father came from North Carolina with General Blackshear and settled in Laurens."
On page 785 is the firt five sentences of the followingparagraph:
"Two pioneer citizens residing in Miller when the county was first opened to settlement, in 1856, both of them men of mark, were Judge Isaac Bush and Dr. Elijah B Bush. The former became the first ordinary of the county, but he resigned this office to enter the State Senate. The latter was a noted surgeon and physician of southwest Georgia. They were half-brothers. The grandfather of these men, William Bush came to Georgia from North Carolina, with the famous General David E Blackshear."