you can flash another ROM but also you can use Preview for Developers... is not Black but you get few features from Black update (also in future your phone get preview updates earlier)
if you have developer Microsoft account, just download
Preview for Developers from Store and activate it...
check for updates
if you dont have dev account, go to
http://apps.windowsstore.com/default.htm
register with your Microsoft account which use on phone
check email on phone and install certificate from mail attachment
go to store and install
Preview for Developers from Store and activate it...
check for updates