Not able to target blog template page with conditional

I have some code in my functions file for the purpose of echoing css classes so I can swap out banners for different pages. It works great on all pages except for my blog index page.

I created a page for the blog index, defined it as blog page in ‘Settings’, and altered index.php to call the function in the banner area. In a conditional statement I’ve tried referencing both the page ID (’99’, as shown below) and also tried using if_page using the page slug, but neither works.

The code:

function is_tree($pid) {
    global $post;
    $anc = get_post_ancestors( $post->ID );
    foreach($anc as $ancestor) {
        if(is_page() && $ancestor == $pid) {
            return true;
        }
    }
    if(is_page()&&(is_page($pid)))
               return true;
    else
               return false;
};

function loadBannerStyles() {
	if(is_tree('105')){ echo 'banner-aboutus';
	} else if(is_tree('92')){ echo 'banner-contact';
	} else if(is_tree('99')){ echo 'banner-newsblog;
	} else { echo 'banner-default'; }
}

This is one of those things that’s so right in front of my nose I can’t see it. How can I code a conditional to change the blog index the way I change all other pages?
Thanks!