BlogAssist is sandboxed, so the data is stored in its sandbox container, within its preference file.

The location is as follows, where "~" means your home directory:

"~ ▶ Library ▶ Containers ▶ com.dejal.blogassist ▶ Data ▶ Library ▶ Preferences ▶ com.dejal.blogassist.plist".

Or as a conventional unix path:

"~/Library/Containers/com.dejal.blogassist/Data/Library/Preferences/com.dejal.blogassist.plist".

If you are using BlogAssist Express from the Mac App Store, replace the "com.dejal.blogassist" with "com.dejal.blogassist.express" in both places, i.e.

"~/Library/Containers/com.dejal.blogassist.express/Data/Library/Preferences/com.dejal.blogassist.express.plist".