It seems that when you have focus in a control on a page, and then click inside the editor's content area, the onBlur function of the other control doesn't work. I have noticed this for the DatePicker and now also for the ComboBox; and I'm sure there's others as well.
Thank you for contacting us regarding this problem.
The DatePicker Blur client event is fired at least on my side, but we must confirm you need a work around for RadComboBox. Here is some markup, which should help: