Are you looking for a C# solution for this? At first glance (in 7.x) I don't see a mechanism in place via the Public Api to set the blog author, and they way you describe it the permissions system may block it. I believe the author is set in the background and it is based on the contextual user when the create command is executed.
Are you implementing some sort of Guest blogging feature and just need to render out who the author is? If so, you may be able to just store some extended post attributes and render them as opposed to actually changing the author property.