"It's an easy transformation to achieve – and one that really makes eye colour pop," says John Frieda's top colourist, Susan Baldwin. "A colourist will put red pigment into the blonde hair to warm up the colour before adding black on top. You only need maybe ten or 20 volume of peroxide, whereas if you're going from dark to light, a colourist is using 30, 40, maybe even 60 volume, which is what is so damaging to hair."
However, if you're going black from dyed blonde, be aware that even though the process requires less peroxide, it's still being applied on top of damage caused by the previous colour application, so hair may end up looking the worse for wear.
Black's also quite a difficult colour to change back from so be sure you can live with it before deciding on the transformation.
"To go lighter from black, you need to lift hair back through the colouring process, which means bringing out the red tones that were added for warmth – and that can also bring out orangey shades, too," Susan points out. "The biggest problem with lightening very dark hair is stopping it from looking ginger, which is why you have to use large amounts of peroxide again."
And that, of course, would be highly damaging.
Susan recommends going gently darker and perhaps adding darker lowlights, to make sure you can live with your new colour.
And don't forget to take a fresh look at your make-up – if your skin tone suited light-coloured hair then there'll be a massive contrast between your complexion and your new dark colour, so use a warmer base and softer colours.