There is a hook you could use called wp_footer. You could add extra attributes to your body tag using pure/vanilla javascript like this:

add_action("wp_footer", "your_theme_adding_extra_attributes");

function your_theme_adding_extra_attributes(){
    ?>
    <script>
        let body = document.getElementsByTagName("body");
        body[0].setAttribute("onBlur", "window.focus()"); 
    </script>
<?php }

Code goes into the functions.php file of your active theme.

This answer has been tested on wordpress 5.8 and works.

Running it on a specific page:

Depending on which page you would need to run it on, you could use different conditional statements. For example:

• Only on archive pages:
  • is_archive() would be a proper conditional check
• Only on the index of your blog:
  • is_home() would be a conditional check.
• Only on front page that may or may not be your blog index:
  • is_front_page() would be a conditional check
• etc...

UPDATE (related to the second request that was not part of the first question)

If you have a page with a slug of blah and you want to modify its body tag, you could use the following snippet:

# The following code would only get executed on yourwebsite.com/blah

add_action("wp_footer", "your_theme_adding_extra_attributes");

function your_theme_adding_extra_attributes(){
    if(is_page("blah")){ ?>
    <script>
        let body = document.getElementsByTagName("body");
        body[0].setAttribute("onBlur", "window.focus()"); 
    </script>
<?php }
}

is_page^Docs

For people who would need to add an extra class NOT an extra attributes, there is a hook called body_class and there are too many answers already on Stackoverflow, such as:

• Add a custom class name to Wordpress body tag?
• body class in WooCommerce

You could hook into the body class filter and do a echo for your attribute, not how filters are meant to work but it gets the job done without js.

add_filter('body_class', 'bt_add_attr_to_body');
function bt_add_attr_to_body ($classes) {
    echo 'onBlur="window.focus()"';
    
    return $classes;
}

I noticed from your comments that you would also like to limit to specific pages.
Lets say you want to to happen only on front page.
Youll need to add a if condition for front page, like this

add_filter('body_class', 'bt_add_attr_to_body');
function bt_add_attr_to_body ($classes) {
    if (is_front_page()) echo 'onBlur="window.focus()"';
    
    return $classes;
}

Based on where you want this attribute to appeare, you will need to create the appropriate condition.