I am running Magento and a WordPress blog on Nginx. URL rewriting was working fine on Magento, but whenever I added a location block for /blog/ with the rewrite rule in it, I got a 403 error for the blod directory.

It turns out that this is because you have redeclare the default files for this directoy, then do the rewrite, like so:

location /blog/ {
index index.html index.php;
expires 30d;
if (!-e $request_filename) {
rewrite . /blog/index.php last;
}
}