Some of Microsoft's own tooling has the ability to decompile XBF files (interesting). You can leverage it yourself as was done by the application at the following link:
https://xamldecompiler.codeplex.com/
Whether Microsoft publishes the format details or not, the format is now out in the wild. If you can parse through c++ headers, the details are laid out here:
https://github.com/WalkingCat/XbfDump