We belive that Rev. John is the brother of Rev. Joel. They purchased land together in Rutherford Co. NC when they first arrived.
MY Joel Blackwell (ca 1738-1821) is known to have a brother which is documented as Isaiah. It is believed that they had a brother, Daniel, as the three of them are in Orange Co. NC in the 1760's. The SC RW Bounty Land grant says that Zachariah is the heir of Daniel. As far as we know there is no relationship to Rev. Joel Blackwell.
Rev. Joel Blackwell has a tombstone in Henderson Co. NC. You can see it on findagrave.