Try this:
<TextBlock> line1 <LineBreak /> line2</TextBlock>
I know this is ressurecting an old question, but I had the same problem. The solution for me was to use HTML encoded line feeds ( ).
Line1 Line2
Looks like
Line1 Line2
For more of the HTML encoded characters check out w3schools
The easiest way is
<TextBlock> blabla <LineBreak /> coucou <LineBreak /> coucou 2 </TextBlock>
So you just write XAML code, and the <LineBreak /> has exactly the same meaning the in HTML or the "\n" in C#.
<LineBreak />