Hello Wayne,

How much do you want to split the ribbon up?  A XamRibbon can have n number of tabs and each tab can have n number groups which in turn can contain n number of controls.  Depending on your requirements it can get quite complicated trying to split things up.  At the simplest, you would only be interested in splitting each tab into it's own file which you can do using the method you already showed at the bottom of your post.  Essentially you derive your own class from RibbonTabItem which will go into it's own XAML and CS files.  Inside the XAML you define how the tab would appear and then in your Ribbon you can add instances of these derived classes to the Tabs collection.

I'm no expert in Blend but I don't see why this approach wouldn't allow you to use it.  While blend won't be able to render each individual derived RibbonTabItem, you will be able to use it to add RibbonGroups and controls to the tab and modify their properties.  Once you add them to the ribbon though, they will be rendered.