Probably the easiest thing for you to do is insert a two of Next Page
Section Breaks before each of the first, the twenty fifth and the fiftieth
pages, then unlink all the headers and footers if they are linked, delete
them from the blank page that will be located between each of the three sets
of Next Page Section Breaks and in the Format Page Number dialog when on
what was the page to be numbered as one, format the page numbering to start
at 1; when on the page that was to be numbered 25, format the page number to
start at 25; and similarly when on the page that was to be numbered 50,
format the page number to start at 50.

It depends what you mean by blank pages. If you have pages with text that
you want not to be numbered, then you can usually achieve this with the
"Different first page" setting (Layout tab of Page Setup). If you insert a
section break before the "blank" page, then it will have the First Page
Header/Footer, which can be left empty. If you want a totally blank page,
then you can use an Odd Page (or, more rarely, Even Page) section break,
which will insert a blank page, if needed, when the previous section ends on
odd (or even) page.

I think you probably need to (re)read and comprehend
http://word.mvps.org/FAQs/Numbering/PageNumbering.htm and
http://word.mvps.org/FAQs/Formatting/NumberingFrontMatter.htm