You need to use RenderTransformOrigin=".5,.5"
<Grid>
<TextBlock Name="textBlock1" RenderTransformOrigin=".5,.5" HorizontalAlignment="Center" VerticalAlignment="Center" TextWrapping="Wrap" FontSize="25" Foreground="#FFF63AFF" FontWeight="Bold">
TextBlock
<TextBlock.RenderTransform>
<TransformGroup>
<RotateTransform Angle="45" />
</TransformGroup>
</TextBlock.RenderTransform>
</TextBlock>
</Grid>
Output