我不知道从提供的代码中我对您的任务理解了多少，但是请看这个实现变体。

IoComponentViewModel：

public class IoComponentViewModel : INotifyPropertyChanged
{
    public static IoComponentViewModel Instance { get; set; }

    public event PropertyChangedEventHandler PropertyChanged;

    private string _xState;

    public string XState
    {
        get { return _xState; }
        set
        {
            if (_xState == value)
                return;

            XStates.Add(_xState = value);
            OnPropertyChanged($"XState");
            
        }
    }

    public ObservableCollection<string> XStates { get; } = new ObservableCollection<string>();

    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

XAML：

<Grid x:Name="PART_Grid">
    <Grid.DataContext>
        <local:IoComponentViewModel/>
    </Grid.DataContext>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition/>
    </Grid.RowDefinitions>
    <!--<Label>-->
        <TextBlock x:Name="Label1" TextWrapping="WrapWithOverflow"
                   Text="{Binding XState, Mode=OneWay}">
        </TextBlock>
    <!--</Label>-->
    <ListView Grid.Row="1"
         x:Name="ListView"
             ItemsSource="{Binding XStates}" />
</Grid>

代码隐藏：

//public ObservableCollection<string> Messages { get; set; }
    public UserControllerIo()
    {
        //Messages = new ObservableCollection<string>();
        InitializeComponent();
        // IoComponentViewModel.Instance = new IoComponentViewModel();
        //Label1.DataContext = IoComponentViewModel.Instance;
        IoComponentViewModel.Instance = (IoComponentViewModel)PART_Grid.DataContext;
        //Messages.Add(Label1.Text);
    }

我最初误解了这个问题。有两个问题。您的列表没有绑定到视图模型，因此您需要一个元素引用。

<UserControl x:Class="StackOverflow.UserControllerIo"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:StackOverflow"
         mc:Ignorable="d" 
         d:DesignHeight="450" d:DesignWidth="800"
         x:Name="MyUserControl"
         >
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition/>
    </Grid.RowDefinitions>

    <Label>

    <TextBlock Foreground="Black" x:Name="Label1" TextWrapping="WrapWithOverflow"
               Text="{Binding Path=XState, Mode=OneWay}">
    </TextBlock>
    </Label>

    <ListView  Grid.Row="1"
        x:Name="ListView"
        ItemsSource="{Binding Messages, ElementName=MyUserControl}" >

    </ListView>
</Grid>

其次，在您将Label1.Text添加到数据绑定的时候，它还没有准备好。因此，在读取文本之前，您需要等待绑定，例如在load事件中，如下所示：

public partial class UserControllerIo : UserControl
{
    public ObservableCollection<string> Messages { get; set; }
    public UserControllerIo()
    {
        Messages = new ObservableCollection<string>();
        InitializeComponent();
        IoComponentViewModel.Instance = new IoComponentViewModel();
        Label1.DataContext = IoComponentViewModel.Instance;

        IoComponentViewModel.Instance.XState = "Something";
        

        Loaded += UserControllerIo_Loaded;

    }
    private void UserControllerIo_Loaded(object sender, RoutedEventArgs e)
    {
        Messages.Add(Label1.Text);
    }
}

编辑：
我的第一个测试误导了我，通过使用int属性来测试在运行时向List添加值。ObservableCollection无论如何都会更新！

问题在于如何声明Messages属性。如果控件上有一个属性，则它需要是一个依赖属性来通知UI。
替换

public ObservableCollection<string> Messages { get; set; }

与

public ObservableCollection<string> Messages
{
  get { return (ObservableCollection<string>)GetValue(MessagesProperty); }
  set { SetValue(MessagesProperty, value); }
}

public static readonly DependencyProperty MessagesProperty =
    DependencyProperty.Register("Messages", typeof(ObservableCollection<string>), typeof(UserControllerIo), new PropertyMetadata(null));

你应该没事的
或
你可以在你的UserControl类上实现INotifyPropertyChanged。
别忘了维护@Clemens关于绑定的评论！！！
ItemsSource="{Binding Messages, RelativeSource={RelativeSource AncestorType=UserControl}}