Redirect *.php to clean URL

I think you just need to change your last line to be:

RewriteRule ^(.*)$ /blah/index.php?/$1 [L]

I have no specific experience with CodeIgniter but generally frameworks want to push all requests through a central controller (index.php) and then parse the request's URL from within there.

EDIT: Just updated my answer with the example from here: http://codeigniter.com/wiki/mod_rewrite/, that should be how CodeIgniter wants it.

UPDATE: You should probably duplicate your RewriteConds for the second RewriteRule, they only apply to the first:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)\.php$ /blah/$1 [R=301,L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /blah/index.php/$1 [L]

Thanks @Cryo for his help, this is the fix:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^(.*)\.php$ /blah/$1 [R=301,L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^(.*)$ /blah/index.php/$1 [L]

What about:

RewriteRule ^(.*)(\.php)?$ /blah/index.php/$1 [L]

and remove the previous line.

Also make sure that /blah/about.php does not exist.