Gotta agree with Shnooka30 on this one. Because blogs may be identified by topic rather than author, it would only make sense that a custom avatar could be assigned to the blog to override the blog creator or default avatar.
BuddyPress is a volunteer project; other than the lead developer who has a portion of his time dedicated to developing and managing BuddyPress, no one else is paid to code it. The project heavily relies on the contributions of others.
That being said, it is not too hard to code a blog avatars plugin for BuddyPress, someone just needs to find the time out of their schedule to code it. Be it for a client or for free.