Semantically, there is not a blank line, there is, at most, some "break" in the story, a change in scene, an unnamed section break, whatever. And to mark this you can either:
Code:
<p>This is a normal paragraph.</p>
<p class="scene-break">This is a paragraph after a break.</p>
and put some top margin in p.scene-break, or:
Code:
<p>This is a normal paragraph.</p>
<div class="scene-break"/>
<p>This is a paragraph after a break.</p>
and style the div.scene-break to have some height, margin, content, etc. (depending on the renderer, you may have to put an inside).
And of course there will be cases where this is not rendered exactly the same with and without CSS, that's the whole point of CSS and content/formatting separation. Anyway, this is ePUB, and ePUB readers are required to support CSS.