With Apache FOP, in order to output a space like   in HTML you would have to insert  .
So if you want to insert a blank line, you just have to write a fo:block whose contents is   like this
<fo:block> </fo:block>
Edit
  always worked for me, but you can also try with   to see if it works.
Otherwise you can use fo:leader to output a blank line, like this
<fo:block>
    <fo:leader />
</fo:block>
<fo:leader> without any attributes will generate an empty line that will fit the whole page width, generating an empty line.
There is also an other way which involves specifying the amount of space needed before your block of text with the space-before attribute, like this
<fo:block>
    <!-- your header -->
</fo:block>
<fo:block space-before="1pt">
    blah blah
</fo:block>