Trying to style a textblock that's used in a validationrule

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I am working on XAML user control to display validation errors using a ValidationRule and a data template to display the validation error to the user. I thought putting in a textblock, styling it with red as the foreground color, etc., would have been sufficient. It turns out that I was wrong about that. Apparently, any control within a data template cannot be simply styled. I found this link on Stack Overflow that looked very promising to resolve my issue: WPF Some styles not applied on DataTemplate controls.

What I have tried:

Following the answer that starts with, "I discovered a simple workaround..." to the post on SO I gave, I have this code:

<StackPanel Orientation="Horizontal">
	<StackPanel.Resources>
		<DataTemplate DataType="{x:Type ValidationError}">
			<DataTemplate.Resources>
				<Style TargetType="TextBlock" BasedOn="{StaticResource {x:Type TextBlock}}" />
			</DataTemplate.Resources>
			<TextBlock Text="{Binding ErrorContent}" 
					   Foreground="Red"
					   VerticalAlignment="Center"
					   FontWeight="Bold"/>
		</DataTemplate>
	</StackPanel.Resources>
	<TextBox x:Name="MaxProficienciesTextBox"
		 Style="{StaticResource LeftAlignTextBoxStyle}"
		 Margin="0,5,3,5"
		 MaxLength="2"
		 IsEnabled="{Binding IsMaxProficienciesEnabled}" 
		 KeyUp="MaxProficienciesTextBox_KeyUp" Loaded="MaxProficienciesTextBox_Loaded">
		<TextBox.Resources>
			<val:BindingProxy x:Key="proxy" Data="{Binding}" />
		</TextBox.Resources>
		<TextBox.Text>
			<Binding Path="MaxProficiencies"
				 UpdateSourceTrigger="PropertyChanged" ValidatesOnExceptions="True">
				<Binding.ValidationRules>
					<val:MaxProficienciesValidationRule Min="1">
						<val:MaxProficienciesValidationRule.Wrapper>
							<val:MaxProficienciesValidationRuleWrapper SolutionID="{Binding SolutionID}" />
						</val:MaxProficienciesValidationRule.Wrapper>
					</val:MaxProficienciesValidationRule>
				</Binding.ValidationRules>
			</Binding>
		</TextBox.Text>
	</TextBox>
	<ContentPresenter Content="{Binding ElementName=MaxProficienciesTextBox, Path=(Validation.Errors)[0].ErrorContent}" />
</StackPanel>

I'm getting an error on the {x:Type TextBlock} style in the DataTemplate.Resources. I don't understand why. Please show me what I've done wrong.