It's been another year. I want this. In general, I want to be able to set defaults cosmetic properties for all block diagram elements.
There is no Option to change the BG colour of all free Labels. There is an Option to change the Default Label colour of both Front panel & Block diagram. To do so go to respective Panel either Front panel or Block diagram Click the Font options environment in the Panel. Then click the Font dialog Box and will Popup a Dialog box. where we can set the Default Colour, Size, Font Style for all the Controls and Indicators With Free labels. To set the Background Colour you need to use Property node of respective Controls and Indicators.