As explained at https://en.support.wordpress.com/avatars/blavatars/ , the Blavatar is a picture / icon representing the blog on wordpress.com. Right now, it is represented only by the default WordPress logo, see e.g.:
https://en.wordpress.com/tag/wikipedia/

According to the linked documentation, "Blavatars look best if the image you’re using is square and at least 128 x 128 pixels in size", and are also reused as favicon, although we have already set the favicon separately in the theme. The Wikimedia logo seem like the obvious candidate. It can be set in the blog admin interface without changing the theme.

Setting a blavatar may also help with setting a feed icon (T70987), but it's not quite clear to me if that is derived from it.