Is it possible to bold only a portion of the text in a TextBlock GraphObject? If not, how would you recommend accomplishing this?
No, the TextBlock.font property, like the other TextBlock properties, applies to the whole text rendered for the TextBlock.text string.
Depending on the situation, it might be possible to position three TextBlocks in a row. Or more generally but less efficiently, render some simple, restricted HTML as an image to be shown by a Picture: Rich Text Rendering