Almost empty .htaccess file causing 500 error response

I suspect that you might be able to avoid the infinite loop in your second ruleset by putting the rewriting directives in the main virtual host configuration, not in an .htaccess file. That's generally good practice anyway, since rewriting runs a lot faster when you put it in the virtual host, and you can get further speed benefits if you can tell Apache to ignore .htaccess files altogether.

The logic behind this is explained in the mod_rewrite technical documentation. Here's the gist: as you know, rewrite rules can change which filename a given request corresponds to, and even which directory the resulting file is in. In fact, your

RewriteRule ^$ /blog/

is a perfect example, which changes the document root to the blog directory. But Apache needs to know the directory in order to figure out which .htaccess files to check for the request. Maybe you can see the problem here: by the time Apache reaches your .htaccess file, it has already determined the name of the file it should be accessing, which means it's too late for your rewriting rules to apply.

Apache internally solves this problem by creating a new subrequest, which starts processing over from the beginning, and injecting the rules from your .htaccess file into the processing stream so they will apply to the request. As a side effect of this, all applicable rewrite rules are applied again, and this means that the [L] flag is kind of a lie when you use it in a .htaccess file. So even though you put [L] on your rule to try to force it to be the last rule applied, it really isn't; all the rules are applied again when Apache processes its internal subrequest.

If you don't have access to the main server configuration, you might be able to do this:

RewriteRule ^/blog/?$    /           [R,L,NS]

The NS flag prevents the rule from being applied to subrequests.

Within your virtual host conf (or httpd.conf/apache2.conf) you can use the ErrorDocument directive, as follows:

ErrorDocument 500 /errordocs/500error.php

It's inadvisable to do too much in your 500 error page, as you could hit the same error condition that caused the 500 response in the first place!