I’m posting it here because it seems like a ‘forgotten feature’. Buttons have a WidgetStyle but TextBlocks do not.
However, in the UE4-editor I can make a SlateWidgetStyle for the TextBlock (containing the font and color and such). But neither C++ nor Blueprints have any way to apply it without me manually creating it myself in C++ (which involves quite a bit of hacking around). Was this forgotten for TextBlock? I asked someone else with a lot of UE4 experience and he told me it was likely something the UE4-team forgot to implement. He checked the source and it was missing.
See also: 'WidgetStyle': undeclared identifier - UI - Unreal Engine Forums