At my work with Expression Blend 2 May Preview i run into a small issue that the right click menu -Bring to back- doesnt work in some scenarios. Against first idea, it's not a attribute and you have no layers (it is also not a shape)
The simples solution is that the first object in XAML is behind all following.
Solution: switch to XAML view and move object to begin of document.
Simalar way is on grouping objects. Everything within a Canvas is grouped.