The content width of the Blurb module is set to 550px by default.
To adjust the content width of the Blurb module to 100%, follow these steps:
Open the Blurb module settings.
Navigate to Design > Sizing.
Locate the "Content width" option and change it to "100%."
To set this width as default, edit the default preset and set 100% for the Content Width:
More details about presets can be found here: Global Presets