TextBox transparente para Windows Phone

Abaixo está um estilo que, aplicado ao seu textbox, vai deixá-lo totalmente transparente. A diferença entre aplicar este estilo e simplesmente colocar a propriedade Background do textbox como transparente, ou removê-la por completo, é que este estilo também remove o efeito de foco que o textbox recebe quando é tocado. Não importa qual cor de fundo você defina, quando o textbox recebe foco o fundo fica branco.

Aqui vai o estilo (você precisa adicionar isso aos resources do seu app/page/user control):

<ControlTemplate x:Key="PhoneDisabledTextBoxTemplate" TargetType="TextBox">
	<ContentControl x:Name="ContentElement" BorderThickness="0" HorizontalContentAlignment="Stretch" Margin="{StaticResource PhoneTextBoxInnerMargin}" Padding="{TemplateBinding Padding}" VerticalContentAlignment="Stretch"/>
</ControlTemplate>
<Style x:Key="TransparentTextBoxStyle" TargetType="TextBox">
	<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilyNormal}"/>
	<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeMediumLarge}"/>
	<Setter Property="Background" Value="{StaticResource PhoneTextBoxBrush}"/>
	<Setter Property="Foreground" Value="{StaticResource PhoneTextBoxForegroundBrush}"/>
	<Setter Property="BorderBrush" Value="{StaticResource PhoneTextBoxBrush}"/>
	<Setter Property="SelectionBackground" Value="{StaticResource PhoneAccentBrush}"/>
	<Setter Property="SelectionForeground" Value="{StaticResource PhoneTextBoxSelectionForegroundBrush}"/>
	<Setter Property="BorderThickness" Value="{StaticResource PhoneBorderThickness}"/>
	<Setter Property="Padding" Value="2"/>
	<Setter Property="Template">
		<Setter.Value>
			<ControlTemplate TargetType="TextBox">
				<Grid Background="Transparent">
					<VisualStateManager.VisualStateGroups>
						<VisualStateGroup x:Name="CommonStates">
							<VisualState x:Name="Normal"/>
							<VisualState x:Name="MouseOver"/>
							<VisualState x:Name="Disabled">
								<Storyboard>
									<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="EnabledBorder">
										<DiscreteObjectKeyFrame KeyTime="0">
											<DiscreteObjectKeyFrame.Value>
												<Visibility>Collapsed</Visibility>
											</DiscreteObjectKeyFrame.Value>
										</DiscreteObjectKeyFrame>
									</ObjectAnimationUsingKeyFrames>
									<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="DisabledOrReadonlyBorder">
										<DiscreteObjectKeyFrame KeyTime="0">
											<DiscreteObjectKeyFrame.Value>
												<Visibility>Visible</Visibility>
											</DiscreteObjectKeyFrame.Value>
										</DiscreteObjectKeyFrame>
									</ObjectAnimationUsingKeyFrames>
								</Storyboard>
							</VisualState>
							<VisualState x:Name="ReadOnly">
								<Storyboard>
									<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="EnabledBorder">
										<DiscreteObjectKeyFrame KeyTime="0">
											<DiscreteObjectKeyFrame.Value>
												<Visibility>Collapsed</Visibility>
											</DiscreteObjectKeyFrame.Value>
										</DiscreteObjectKeyFrame>
									</ObjectAnimationUsingKeyFrames>
									<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="DisabledOrReadonlyBorder">
										<DiscreteObjectKeyFrame KeyTime="0">
											<DiscreteObjectKeyFrame.Value>
												<Visibility>Visible</Visibility>
											</DiscreteObjectKeyFrame.Value>
										</DiscreteObjectKeyFrame>
									</ObjectAnimationUsingKeyFrames>
									<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="DisabledOrReadonlyContent">
										<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneTextBoxReadOnlyBrush}"/>
									</ObjectAnimationUsingKeyFrames>
								</Storyboard>
							</VisualState>
						</VisualStateGroup>
						<VisualStateGroup x:Name="FocusStates">
							<VisualState x:Name="Focused"/>
							<VisualState x:Name="Unfocused"/>
						</VisualStateGroup>
						<VisualStateGroup x:Name="ValidationStates">
							<VisualState x:Name="InvalidFocused"/>
							<VisualState x:Name="Valid"/>
							<VisualState x:Name="InvalidUnfocused"/>
						</VisualStateGroup>
					</VisualStateManager.VisualStateGroups>
					<Border x:Name="EnabledBorder" BorderThickness="{TemplateBinding BorderThickness}" Margin="{StaticResource PhoneTouchTargetOverhang}">
						<ContentControl x:Name="ContentElement" BorderThickness="0" HorizontalContentAlignment="Stretch" Margin="{StaticResource PhoneTextBoxInnerMargin}" Padding="{TemplateBinding Padding}" VerticalContentAlignment="Stretch"/>
					</Border>
					<Border x:Name="DisabledOrReadonlyBorder" BorderThickness="{TemplateBinding BorderThickness}" Margin="{StaticResource PhoneTouchTargetOverhang}" Visibility="Collapsed">
						<TextBox x:Name="DisabledOrReadonlyContent" Foreground="{StaticResource PhoneDisabledBrush}" FontWeight="{TemplateBinding FontWeight}" FontStyle="{TemplateBinding FontStyle}" FontSize="{TemplateBinding FontSize}" FontFamily="{TemplateBinding FontFamily}" IsReadOnly="True" SelectionForeground="{TemplateBinding SelectionForeground}" SelectionBackground="{TemplateBinding SelectionBackground}" TextAlignment="{TemplateBinding TextAlignment}" TextWrapping="{TemplateBinding TextWrapping}" Text="{TemplateBinding Text}" Template="{StaticResource PhoneDisabledTextBoxTemplate}"/>
					</Border>
				</Grid>
			</ControlTemplate>
		</Setter.Value>
	</Setter>
</Style>

Com o estilo pronto, basta aplicá-lo ao seu textbox assim:

<TextBox Style="{StaticResource TransparentTextBoxStyle}"/>

E é só isso. Seu textbox deve estar 100% transparente agora. Caso queira testar antes de usar, fiz um projeto de exemplo que você pode baixar aqui.