Whilst blogger does have an api of sorts, (via a http: request) I’m not certain how to include this in a known add-in (basically I’d probably rather wait for a more savvy developer to work it out). I have found it is pretty simple to share your post (with a picture) into blogger, by firstly sharing it to flickr (which is one of the current social media share links built in.)
The sharing process involves two steps in flickr, and three or four steps in blogger. Thus I have found it best to have both flickr and blogger open in separate tabs in your browser. To begin open the photo page for the image uploaded from Known, click on the share icon (which is the right facing curved arrow on the right hand side of the image. This brings up the share dialogue, click on the HTML radio button and copy the HTML code in the bottom dialogue line (see example on the right). This is HTML code that will display the photo in blogger while the photo itself stays on flickr. Now go to the blogger page and select the Create New Post (it is a pencil icon up on the top left, but I’m sure if you blog you already know that). Firstly change the edit mode from compose to HTML on the blogger post page. Now paste in the HTML code from flickr into the post text area,(see the example below in the blue dashed line highlight box). Next return to the flickr page and click on the small edit icon (again a pencil icon) beside the title and comment section. This will them show you the actual HTML version of the text from your know post (see example on the left). Copy this and return to the blogger page and post it under the image HTML (also shown within the green box below). You know have a post almost identical to the post back in known. unfortunately like the google+ link any hashtags will not be automatically transferred so if you want to use any you will need to enter them manually using the lables button on the right hand side of the blogger page.
Perhaps this is not as nice as an automatic share, and it doesn’t add a flag back into your Known post that it is also posted in blogger but it is pretty simple once you understand the sequence, and it still has the link to your original Known post to establish your ownership and provenance.