Is there any way to mark a blog or a single post as private on Blogspot?
Put the posts that you want to be private into a separate blog, and make it private (Settings > Permissions).
If you want, put links to the private blog into the public one, along with comments about who is eligible to see it.
Just remember that private blogs may not be as secure as you think - see here for details: http://blogger-hints-and-tips.blogspot.com/2008/02/restricting-your-blogs-readers-isnt-as.html
No, this is not possible.
It's not a features or manpower issue, its a server load issue, and it would affect all of us that publish or read blogs on Blogger / Google servers (ie, all bloggers). Having to check authorisation to access each individual post, for each viewer of our blog, would require code complexity, and cause unacceptable server load, and our blogs would load slower. Slower blog loading == less readers
You can only mark the whole blog as private or public, not individual posts.
I added the feature request on the Google forums for Blogger but I have seen it has been asked before.
It's a pity Blogger does not support it as WordPress, Tumblr and Posterous supports private posts.
For Blogger the only workaround would be to save the post as draft.