No notification when a DependencyProperty value has changed?

I am working on a FrameworkElement called Plot which has to dynamically
create TextBlocks. All of them should use the same font size, font family,
foreground and so on.

Instead of giving my plot class members which hold all these properties for
the text blocks I just want to have a single TextBlock as member. This
element is never added to the visual tree but serves as a property storage
medium and is used when new text blocks are created.

Now I want all my text blocks to be updated if some of the properties of the
"template" text block is changed. But no way. I do not get any notification
of that textblock if for example the font size is changed.

What is going wrong?