Newline in string attribute
<TextBlock Text="Stuff on line1
Stuff on line 2"/>
You can use any hexadecimally encoded value to represent a literal. In this case, I used the line feed (char 10). If you want to do "classic" vbCrLf, then you can use 

By the way, note the syntax: It's the ampersand, a pound, the letter x, then the hex value of the character you want, and then finally a semi-colon.
ALSO: For completeness, you can bind to a text that already has the line feeds embedded in it like a constant in your code behind, or a variable constructed at runtime.
May be you can use the attribute xml:space="preserve" for preserving whitespace in the source XAML
<TextBlock xml:space="preserve">
Stuff on line 1
Stuff on line 2
</TextBlock>