exclude category from certain page

Hello,
I put a function inside function.php:

if ( is_page('1237') ) {
function exclude_category( $wp_query ) {
    // Add the category to an array of excluded categories. In this case, though,
    // it's really just one.
    $excluded = array( '-18' );
    // Note that this is a cleaner way to write: $wp_query->set('category__not_in', $excluded);
    set_query_var( 'category__not_in', $excluded );
}
add_action( 'pre_get_posts', 'exclude_category' );
}

the function works without the if statement, while with the if the target page continues to show the category 18.

The page 1237 loads the blog template.

how can I modify the if statment to activate the function only on that page?