WPF Popup UI showing black

You need to set the AllowsTransparency="True" Popup Property to True

Here is an example:

<Grid>
    <StackPanel>
    <Button Click="Button_Click" Width="100" Height="20" Content="Click" />
        <Popup x:Name="popup" Width="100" Height="100" AllowsTransparency="True">
            <Grid Background="Transparent">
                <TextBlock Text="Some Text" />
            </Grid>
        </Popup>
    </StackPanel>
</Grid>

and the click handler

private void Button_Click(object sender, RoutedEventArgs e)
{
    popup.Visibility = System.Windows.Visibility.Visible;
    popup.IsOpen = true;
}