Or without the need for an additional user interface element:
<Grid Background="#80000000" Opacitiy="0.5">
<Grid.RowDefinitions>...</Grid.RowDefinitions>
<Image Source="..." Opacity="1.0" Grid.Row="0"/>
<TextBlock Text="..." Opacitiy="1.0" Grid.Row="1"/>
</Grid>
, "# 80000000" Alpha () = "80", = "00" , = "00" , = "00"