It depends on the blog, blog owners can determine this. As a visitor to a blog, you will be notified when you try to comment if restrictions apply. More information is gleened from a profiled blog-city user and so this is why a blog owner may choose to receive only comments from blog-city profile holders. It also alieviates spam.

To find out how to set comments please visit Comment Options This is found under Settings, Blog Entry Options on the left hand menu.
We do recommend signing up for an account. It's free, it only required email/username and password minimally, and it has many advantages. For example, when you make a comment, there are less steps to complete the comment. The cookie is persistent so you are free to edit it again whenever you want. In addition to this, you can optionally receive email alerts as to when that blog-entry updates. You only need to register once; allowing you to leave comments on any blog site hosted at Blog-City.
If you are logged in - note less fields to complete.
If you are logged out - there are more security steps to go through