Hi Frank,
We have an Apache document up that you could follow:
http://mongrel.rubyforge.org/docs/apache.html
And, then you would either create a new virtual host for your blog (like blog.rails.com) or you can use the --prefix option for mongrel and put the site at /blog. The virtual host is what rails-core does, so that's probably what you should do.
After that it's just a matter of setting up the second mongrel/rails configuration to run your typo install.
