Take a look at the formatting of the paragraph that appears immediately
after each blank page.  Is it set to "Page Break Before"?

The blank pages are caused by "Odd page" section breaks. If you did not
explicitly choose these breaks (that is, if you inserted "Next page"
breaks), then Word may have changed them behind your back because you are
trying to put a portrait page on the back of landscape or vice versa. The
reason for this is that printers evidently find it much harder to print
landscape than portrait and especially difficult when duplexing. To
accommodate the printer, Word changes the break type. You can overrule Word
(usually) by going to the Layout tab of Page Setup and changing the section
start back to "New page."

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Suzanne S. Barnhill
Microsoft MVP (Word)