Our IntegrationAPI.h exposes a function to define the default properties for text elements:

void SetFontDefaultProperties(float size, FontWeight weight, FontStretch stretch, FontStyle style);

This is what TextBlock properties use as defaults when no local value is set or inherited from a Control container. If you change these settings in your editor, you should call it in the runtime too so the result is the same.

Changing the default Foreground color is not a trivial task though. The appropriate color would depend on the background of the container. And it is a property that is not usually set but inherited from the Control container. For example, you would set the Foreground in the Button, but not in a TextBlock placed inside its template.