How to block access to blog-page for users not logged in?

You can do this way

function block_access() {
  if (is_page (blog) && !is_user_logged_in ()) {
    $loginUrl = home_url('/login-page/');
    wp_redirect($loginUrl);
    exit(); 
  }
}
add_action( 'template_redirect', 'block_access' );