Hi there, thanks for the report.
A 404 not found error on the blog page usually happens when the front page was set to display a static page that was later deleted. Could this be the case for the sites you saw?
This was part of core WordPress behaviour, but was recently fixed in this Trac ticket. It will be part of WordPress 3.7 and will also make its way to WordPress.com:
I'm seeing the footer credits on a test Sunspot theme with no posts:
If you do run across either of these issues again, feel free to let me know the URLs and I'll be glad to have a look. Thanks.