On page editing of properties in MVC works so that when a property value has been updated a request is made to PropertyRender Controller (you can see this request from browser if you open up developer tools). That controller will then basically render a view which does @Html.PropertyFor(theProperty). So in your case will it call the PropertyRender Controller that it should render property 'Callout' (since that is what you specify in EditAttributes). So you need to have a partial template for Callout (that basically could call render CalloutContent) that can be used during editing.

This is not how on-page-edit works. I would recommend you to change your code to something like this:

@Html.PropertyFor(m => m.CurrentBlock.Callout)

Then create a view called Callout in /views/shared/blocks/Callout.cshtml. And in this view you put:

@model Callout

<p>
   @Model.CalloutContent<br />
</p>

If you already have a view for the block, then you can use a tag and create a view for that tag instead:

@Html.PropertyFor(m => m.CurrentBlock.Callout, new { Tag = "MyTag" })

And name the view MyTag instead /views/shared/displaytemplates/MyTag.cshtml

Lucy, Can you please clarify if Johan B's solution works, or Johan P's?  or both?