It sounds like in your parent application (in the root), you have a web.config file for that parent application.
In that web.config file (in the root), on line 44, it's setting the ASP.NET theme to "myTheme". Do you see this on line 44?
BE.NET has its own theme system. ASP.NET also has its own theme system that uses the App_Themes folder. You don't need an App_Themes folder in the /blog sub-folder.
Are you getting this error when accessing the parent application, or when accessing the /blog application?
If you're getting this error while trying to access the /blog folder, this could be because the settings (theme and possibly other settings too) in the parent application are propagating down to the child applications (/blog). What you can try doing
is in the parent application web.config file, wrap the existing <system.web> section in the following <location> tag.
<location path="." inheritInChildApplications="false">
............ existing settings ...........
Again, these <location> tags above should be put into the PARENT web.config file (not the BE web.config file).
Along the same lines, if you have a <system.webServer> section in the parent web.config file, I would try wrapping the <system.webServer> section in the same <location> tag as shown above.