On mine it was the black one, but my brake lights weren't working at all.
Just a guess but I think the blue one might be for disengaging the cruise control.
I suppose you could test by removing the electrical connector and see if the lights go off. The switch itself is easy to remove, give the body of the switch roughly a quarter of a turn and it pops out.
2005 Fiesta ST
Milltek non-res, Newman fast road cams, Fusion Fabs custom inlet manifold