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>