NOTE: This solution is no longer required, please follow the instructions here which will fix this issue and also an error when trying to update pages.
In the Divi 3.0.70 update, there were some changes to main-modules.php which means your blog grid may now show as fullwidth.
In order to fix this you need to update the cbm.php file included with the Clarity child theme.
Note: You will need FTP access to apply this fix and please backup your site first as you will be changing PHP files.
The attached cbm.php file contains code from the blog module in Divi version 3.0.72 so please update your site to this Divi version prior to applying this fix.
Using your FTP program replace the cbm.php file located in Clarity > custom-modules > cbm.php with the one attached here.
Clear your cache and your blog module should revert back to a grid layout.