This should occur unless you have deleted browser history, turned off cookies or taken any other action which would prevent your computer from remembering your last sign-in.
You should select Design if the e-mail address displayed is the one use for your Blogger account.
However; in the same area you may also see an e-mail address other than the one you use for your Blogger account. This means someone else (or yourself) has logged into a different Blogger or any other Google account, using the same browser on the computer (device) you are using.
Select Sign Out and you should be provided a page similar to the following:
If you enter the wrong credentials it will be more than obvious that you have done so and you will be sent to:
Which actually is the page for creating a new Google account. Don't do this unless you wish to create another new Google account.