I got the blog post using:
var manager = BlogsManager.GetManager();
var allPosts = manager.GetBlogs().Where(p => p.Title == "Broan Pro").First().BlogPosts;
var post = allPosts.Where(p=>p.Status==ContentLifecycleStatus.Live).OrderByDescending(p => p.PublicationDate).First()
but post does not have a GetValue() method. Post has a type of Telerik.Sitefinity.Blogs.Model.BlogPost
For the second way you showed I am unsure what the dataItem type should be. I tried
var propDesc = OrganizerBase.GetProperty(typeof (Telerik.Sitefinity.Blogs.Model.BlogPost), "Category") as TaxonomyPropertyDescriptor;
but I wasn't sure where to go from there. I tried to call the GetValue method with the parameter being the BlogPost that I need the categories for but this didn't seem to give what I needed.