I want to show the page title in single post page
What do you mean by “page title of a single post page” The title of a single post page will be the same as the current post being displayed.
The retrieve and display the page title of a single post page, use the native function single_post_title(). Just add it where you need to display the title
…I want to show my page title, in this case “Blog” below my header area in single.php file
Blog cannot be the page title, except when your post’s name is blog. If the name “Blog” is something else or derived from any other source, you’ll need to reveal that source then
If my answer doesn’t answer your question, please file an edit and properly explain what you want.