rounded border on textblock issue - wpf

Question

I am trying to give my textblocks rounded borders. From what I've researched, I should embed a textblock within a border tag and then set the corner radius of the border. For me, it's having the effect of putting a border around the entire row. What am I doing wrong? I set the color of the border to blue to show what is happening. Ideally, I would change it to the same color as the background of the textblock to just give it seamless rounded corners.
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Border Grid.Row="0" Margin="25" Grid.Column="1" Background="Blue"
BorderThickness="1" BorderBrush="Red" CornerRadius="30">
<TextBlock Margin="50" Padding="200,0,200,0"
FontSize="100"
FontWeight="Bold"
Background="Black"
Foreground="White"
VerticalAlignment="Center"
Text="bla bla bla"
HorizontalAlignment="Center">
</TextBlock>
</Border>
</Grid>

Answer 1

Look at the HorizontalAlignment / VerticalAlignment values for both the Grid and the Border. Set whatever is most appropriate for your requirements.
A tool such as Kaxaml is great for playing around with this sort of thing without having to build an entire application.

Answer 2

A better way to do this is to create a copy of the default style for TextBox and modify the border in the ControlTemplate. Here is a simple app that has the default TextBox style extracted and modified to round the corners.
<Window x:Class="WpfApplication1.MainWindow"
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"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<SolidColorBrush x:Key="TextBox.Static.Border" Color="#FFABAdB3"/>
<SolidColorBrush x:Key="TextBox.MouseOver.Border" Color="#FF7EB4EA"/>
<SolidColorBrush x:Key="TextBox.Focus.Border" Color="#FF569DE5"/>
<Style x:Key="TextBoxStyle1" TargetType="{x:Type TextBox}">
<Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"/>
<Setter Property="BorderBrush" Value="{StaticResource TextBox.Static.Border}"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="KeyboardNavigation.TabNavigation" Value="None"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="AllowDrop" Value="true"/>
<Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
<Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<Border x:Name="border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True" CornerRadius="5">
<ScrollViewer x:Name="PART_ContentHost" Focusable="false" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Opacity" TargetName="border" Value="0.56"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="BorderBrush" TargetName="border" Value="{StaticResource TextBox.MouseOver.Border}"/>
</Trigger>
<Trigger Property="IsKeyboardFocused" Value="true">
<Setter Property="BorderBrush" TargetName="border" Value="{StaticResource TextBox.Focus.Border}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsInactiveSelectionHighlightEnabled" Value="true"/>
<Condition Property="IsSelectionActive" Value="false"/>
</MultiTrigger.Conditions>
<Setter Property="SelectionBrush" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightBrushKey}}"/>
</MultiTrigger>
</Style.Triggers>
</Style>
</Window.Resources>
<Grid>
<StackPanel>
<TextBox Margin="10" Style="{DynamicResource TextBoxStyle1}" Text="bla bla bla"></TextBox>
</StackPanel>
</Grid>
</Window>
If you want this style to apply to all textboxes in your app, you should move the style into a resource dictionary and remove x:Key="TextBoxStyle1" from the style definition. It would then apply to all TextBox by default and remove the need to set a Style for each one.