Users who have switched to Blogger in beta will not be able to login to comment on blogs that have not switched. Commenting using the “anonymous” or “other” options will still work. Update (9/12): We have started working on making this possible, however the change is substantial and will take a little while before it is ready. Update (9/19): This has been fixed. You can use the “Sign in with your Google Account” link on the Blogger comment form to log in with your Blogger in beta account.