To introduce additional paragraphs, you can use the LineBreak element:
<UserControl x:Class='SilverlightApplication3.MainPage'
xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
xmlns:d='http://schemas.microsoft.com/expression/blend/2008'
xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006'
mc:Ignorable='d'
d:DesignWidth='640'
d:DesignHeight='480'>
<Canvas xmlns="http://schemas.microsoft.com/client/2007">
<TextBlock Width="450" TextWrapping="Wrap">
this is a test
<LineBreak/>
this is a test
</TextBlock>
</Canvas>
</UserControl>