You can put custom html into about:blank? How do I do that please?
What I mean is adding content dynamically after about:blank loads. You can set contentEditable to "true", inject some innerHTML, use the terrible DOM interface, or make a little JQuery to suit your needs.
You can probably also style about:blank to have whatever you want in it when it loads. I'd imagine that is done differently depending on the browser you're using.
On the other hand, I'm not sure why you'd want to when you can just set your new tab page to a custom page of your choosing.
Edit: clarification
I can add any content I want there.
Then if it phones home I have only myself to blame.