You have to make the formatting the same as the rest of the document.
Then change the Next-Page section break to a continuous section break.
Delete a Next-Page Section Break that Word won't delete
In addition to the steps listed for recording the macro referred to in this, you'll probably have to also display the page background dialog. (I never use backgrounds so did not think of that.)

The thing is, the formatting changes you inserted, but not for the entire document, required a next-page section break. That break is between your content and the blank page. If you could delete that section break, you would undo all of your section formatting that is for the part before the section break.
Showing non-printing formatting marks in Microsoft Word