Default value at design time XAML

Update: Visual Studio 2019 v16.7

You can now do:

<TextBlock Text="{Binding MyText}" d:Text="Design time value"/>

---

If you would prefer a less verbose version of Ian Bamforth's answer, you can just do

<TextBlock Text="{Binding MyText, FallbackValue=None}"/>

Using FallbackValue is wrong, because it also affects the runtime behavior (the fallback value is used if the binding fails to obtain a value from the source).

I came up with a custom markup extension that mimics Binding (ideally I would have preferred to inherit from Binding, but the ProvideValue method is not virtual...):

using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;

namespace MyNamespace
{
    public class BindingEx : MarkupExtension
    {
        private readonly Binding _binding;

        public BindingEx()
        {
            _binding = new Binding();
        }

        public BindingEx(string path)
        {
            _binding = new Binding(path);
        }

        public PropertyPath Path
        {
            get => _binding.Path;
            set => _binding.Path = value;
        }

        public BindingMode Mode
        {
            get => _binding.Mode;
            set => _binding.Mode = value;
        }

        public RelativeSource RelativeSource
        {
            get => _binding.RelativeSource;
            set => _binding.RelativeSource = value;
        }

        public string ElementName
        {
            get => _binding.ElementName;
            set => _binding.ElementName = value;
        }

        public IValueConverter Converter
        {
            get => _binding.Converter;
            set => _binding.Converter = value;
        }

        public object DesignValue { get; set; }

        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            var target = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
            if (target.TargetObject is DependencyObject d && DesignerProperties.GetIsInDesignMode(d))
                return DesignValue;

            return _binding.ProvideValue(serviceProvider);
        }
    }
}

You can use it just like Binding, with the addition of the DesignValue property:

<TextBlock Text="{my:BindingEx Name, DesignValue=John Doe}" />

Note that BindingEx doesn't have all the properties from Binding, but you can easily add them if necessary.

Adapting an example from this question.

This works for me - the text "None" is shown in the designer:

 <TextBlock>
    <TextBlock.Text>
        <Binding ElementName="root" Path="blah" FallbackValue="None" />
    </TextBlock.Text>
</TextBlock>

Hope that helps