Yes, you can change the Default Label to a Custom Label for a guestbook. The Default Label will be:

If you would like, you can add your name and a short message to our Guest Book. Thank you.

To change it, follow these steps:
  1. Navigate to Setup > select Product Configuration
  2. Select the Message Catalog Editor tab
  3. On the left side of the screen, under Application, select Blog from the dropdown
  4. Search for "If you would like"
  5. Click Edit Custom under the Actions column
  6. In the pop up window, enter the custom text
  7. Click Submit