How to use d:DesignInstance with types that don’t have default constructor?

The default constructor is required for a type to be instantiated in XAML. As a workaround you can simply create a subclass of TaskVM that will have the default contructor and use it as a design time data context.

<TextBlock d:DataContext="{d:DesignInstance ViewModel:DesignTimeTaskVM }" 
           Text="{Binding Title}" MouseLeftButtonDown="TextBlock_MouseLeftButtonDown">
</TextBlock>

Another alternative is to set d:IsDesignTimeCreatable to False and a substitute type will be created for you at runtime (using your TaskVM type as a “shape”).

<TextBlock d:DataContext="{d:DesignInstance ViewModel:DesignTimeTaskVM, IsDesignTimeCreatable=False}" 
           Text="{Binding Title}" MouseLeftButtonDown="TextBlock_MouseLeftButtonDown">
</TextBlock>