Just to elaborate on my comment to give some meaning,
From MSDN Docs
Loaded entails that the logical tree that an element is contained within is complete
which helps us since the binding here has a reference to the other TextBlock
element. Bindings also depend on DataContext
and few other factors relative to the specific Binding
but in general they get evaluated after the UI Loads for good reason.
Hence setting the Text
of the source TextBlock
once the UI loads results in everything working fine since Binding
's are active at that point.