When you left Group X, you were made a subscriber to the Blog, instead of an author. So you can’t make posts on Group X’s Blog but you can view it.
If you would like to unsubscribe to a Blog please contact the Blog Author from the black Admin Bar at the top of the screen and request that you be removed.
So far there is no easy way for users to unsubscribe yourself. We may provide this option in a future update.