Atm there are;

Comments:
Open
Closed
Disabled

It would be good if this was changed to (using checkboxes instead of radio buttons);

Comments for:
Everyone
Members
Registered Users
Guests
No One

Now Registered Users excludes Members.
To have both Registered Users and Members the user would check both checkboxes.
Checking Everyone would be equivelant to checking Members, Registered Users and Guests.
Checking No One overides all other checks.

And have a option in; Backoffice->Blogs->ABlog, for the default value for the above options.

I can help with the development of this feature.

Edit:
Updated to apply Guests, and Checkboxes.