Blank HTM file in folders?
 

In almost every folder of the board there is a blank (0 bytes) HTML file. What is the purpose of this?

You will find a blank index.html to avoid people being able to browse directories if webserver permissions are set incorrect.

So are they basically then the same as a 'Error 404 Requested Web Page Not Found" page? or do they need to be completely blank?

No. They are there simply so going into the directory displays a blank page instead of a listing of all the files and folders in the directory. 404 error pages are if the requested file does not exist.