Option 1: Brute Force
For domain VirtualHost you set up in Apache, set up a corresponding VirtualHost for the blog. Something like this:
Option 2: Dynamic Virtual Hosting
The mod_vhost_alias module for Apache allows for Dynamcially configured mass virtual hosting, which allows you to set up one single configuration set that is used for multiple virtual hosts, where everything is identical from domain to domain except the directory where the files will be found.
The secret sauce of dynamic virtual hosting is the
VirtualDocumentRoot directive. It looks something like this:
Which might correspond to --
Check out the documentation for UseCanonicalName to see why it should be present, and directory name interpolation to see what
%0 means, and whether you might want to put something else there instead.
Option 3: Rewrite Rules
Mod_rewrite allows you to supply a set of translation rules which tell Apache to turn one url into another, and then re-route the request to wherever that URL might take it.
With it, you can do pretty much anything, but it takes some mental gymnastics sometimes to understand what you're telling the server to do. I'm not going to go too deep into how to make it work because writing rewrite rules without a solid understanding of how they work is a path that ends ultimately in madness. Best to learn before you leap.
At least skim the mod_rewrite manual, have a good look over the reference page, and read the examples and make sure you understand how they work.