How can I toggle a TextBlock's visibility in a DataTrigger?

You're setting the Visibility on the TextBlock and then trying to override it with a style. That won't work. Try this:

<Window x:Class="TestCollapsed.Views.MainView"    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"    xmlns:c="clr-namespace:TestCollapsed.Commands"    Title="Main Window" Height="400" Width="800">    <Window.Resources>        <Style x:Key="DropDownStyle" TargetType="TextBlock">            <Setter Property="Visibility" Value="Collapsed"/>            <Style.Triggers>                <DataTrigger Binding="{Binding ControlType}" Value="dropDown">                    <Setter Property="Visibility" Value="Visible"/>                </DataTrigger>            </Style.Triggers>        </Style>    </Window.Resources>    <StackPanel>        <TextBlock Text="This is going to be the dropdown control."                   Style="{StaticResource DropDownStyle}"/>    </StackPanel></Window>

I have the same problem. @Bryan's answer is perfect!There are the wrong and right versions.The wrong version:

<TextBlock Text="1999-09-09 16:08" VerticalAlignment="Top" Visibility="Collapsed">                                <TextBlock.Style>                                    <Style BasedOn="{StaticResource TipTextYellow}" TargetType="TextBlock">                                        <Style.Triggers>                                            <DataTrigger Binding="{Binding ElementName=Alcohol,Path=IsFocused}"  Value="True">                                                <Setter Property="Visibility" Value="Visible"/>                                            </DataTrigger>                                        </Style.Triggers>                                    </Style>                                </TextBlock.Style>                            </TextBlock>

The right version:

<TextBlock Text="1999-09-09 16:08" VerticalAlignment="Top">                                <TextBlock.Style>                                    <Style BasedOn="{StaticResource TipTextYellow}" TargetType="TextBlock">                                        <Setter Property="Visibility" Value="Collapsed"/>                                        <Style.Triggers>                                            <DataTrigger Binding="{Binding ElementName=Alcohol,Path=IsFocused}"  Value="True">                                                <Setter Property="Visibility" Value="Visible"/>                                            </DataTrigger>                                        </Style.Triggers>                                    </Style>                                </TextBlock.Style>                            </TextBlock>