To clarify, are the blank pages supposed to have text/graphics and page numbers or are they extra pages?

If there are several missing/blank pages this is a pretty big flaw and it would normally decrease the value quite a bit.  There are people that would still be interested in it, since it is a 1st print and it's a curiosity, but overall it would likely not be worth anywhere near what a 1st print in similar condition would be worth.

This does happen occasionally, but this one might be worse than the typical printing defect/error.

But...

You might be aware of one of the most infamous defects, but if not, here's a link to the 2nd Alpha information:

Dungeon Masters Guide

This is a rare example of a printing defect actually increasing the value significantly.  These particular DM Guides are highly sought after, since they should have all been recalled and corrected.

Have you tried holding the supposedly blank pages up to a lightbulb?

KenH wrote in First Print DM's Guide - blank pages:
No, why? The set of pages follows a regular pattern.

To see if the hidden text is revealed.

10-12 pages is a lot of pages.  It's usually only a couple of pages in the printing errors that I've seen.

The more common printing errors, in rough order of frequency.

1.        Misaligned pages
2.        Ink smudges or smears
3.        Print fading or inconsistency
4.        Print artifacts or blemishes
5.        Binding issues
6.        Creases or folds
7.        Blank pages

Quality control measures have improved quite a bit since these books were produced, so more of these errors are caught now with high-volume publishers.