A Gravatar is a Globably Recognized Avatar (and Profile), a service by Automattic that allows any person to assign an image to an email address. WordPress.com makes extensive use of Gravatars whenever possible, and we recommend using them in your theme and assigning a Gravatar to every author.
To learn how to implement Gravatars in your site theme, please visit Using Gravatars in the WordPress.org codex.
Tip: If you are not seeing avatars on your site, check to make sure Avatar Display is enabled in Settings -> Discussion.
Blavatars are site icons which are automatically used on WordPress.com as the site favicon, iPhone home screen icon, etc.
You can set your site Blavatar under Settings -> General in the site dashboard.
Note: If your theme already contains a reference to an icon in the
header.php file, the blavatar will be ignored.
You can also use the blavatar directly in your theme, with the following functions:
blavatar_exists( $domain )
get_blavatar( $url, $size = '96')
if ( blavatar_exists( blavatar_current_domain() ) ) echo get_blavatar( blavatar_current_domain(), 128);
wpcom_vip_disable_hovercards() in your theme.