You don't need to have the same colours in conduit as with cables: In a cable there will always be a light brown wire to warn you. To avoid problems with wiring in conduit, you could substitute black for red.
Thus, there should never be a situation where you have only blue and black in a new installation.
Had the UK adopted the entire colour code when it changed to green/yellow instead of green, it would have been easy to distinguish new and old by the different earth wires.
