about:blank not always trusted, so not such a good default url for your CRM IFrame

Basically when setting a default page for your Crm Forms IFRAME about:blank can have a trust issues in the browser and it's better practice to use /_static/blank.htm instead.

This nice article from those ascentium guys explains why.
http://xrm.ascentium.com/blog/crm/Post612.aspx