I may not have fully understood the Blocs colour system and Global Swatches in particular.
However, I keep thinking that there should be a non Global Swatches, i.e. a plain Swatches panel that only relates to what element is currently selected and overrides the Global Swatches colour for that element.
E.g. If I want to change a single Header text colour, I would like a way to select it and choose only that Header colour using the non Swatches panel.