How to set a certain Custom Template to a Single Blog Post

There is no build in template assigment for indivual standar posts from edit screen. But you can assign templates for individual posts. There are several options:

Use WordPress template hierarchy system

Under the root folder of your theme you can create this files:

• single.php for all single posts of any type.
• single-post.php, if this file exists, it will be used for standard posts instead of single.php.
• single-{post-type}.php, if this file exists, it will only for {post-type} posts instead of single.php

Use template_include filter

Using template_include filter you can use any template file for single posts that meet your requeriments. For example, use the template template-for-my-post.php for post with ID 124:

add_filter( 'template_include', 'cyb_post_template');

function cyb_post_template( $template ) {
    if ( is_single( 124 )  ) {
        $new_template = locate_template( array( 'template-for-my-post.php' ) );
        if ( '' != $new_template ) {
            return $new_template ;
        }
    }
    return $template;
}