Subdomain URL for blog in Wordpress multisite & Buddypress


I am working in a test platform to allow user to create their own blog.

I am using WP 3.4 Multisite active and Buddypress 1.5.6

When a user register and create a blog, URL of his blog is :

But when I click on this URL I get a blank page.

Do I need to do something in my DNS or apache config ?

Please advise