Getting featured image of blog page rather than post image

This code might be what you’re after. This loops through page templates to grab the featured image. If you add it outside the loop in your page.php template (or index.php, depending on your theme), it should grab the pages’ featured image, not the posts’.

<?php
global $post;
    if ( isset($post) ) {

        //get the ancestors
        $familyTree = get_ancestors($post->ID,'page');
        array_unshift( $familyTree, $post->ID ); //add the current page to the beginning of the list

        //loop through the family tree until you find a result or exhaust the array
        $featuredImage="";
        foreach ( $familyTree as $family_postid ) {
            if ( has_post_thumbnail( $family_postid ) ) {
                $featuredImage = get_the_post_thumbnail( $family_postid, 'full' );
                break;
            }
        }

        // if the page has a featured image then show it
        echo ( $featuredImage ? $featuredImage : "" );

    }    
?>        

You should add this outside your loop