He added: "Sales of black and purple carrots went up by 40 per cent last year. I think people are more open to new ingredients and there has been a huge focus on clean eating too.
"As well as warding off diseases, they are much sweeter and have a slightly different texture."
Dr Charlotte Allender, from the Genetics Resources Unit at Warwick University, said the world's very first carrots were in fact purple and black - but they died out as humans increasingly favoured the orange varieties. She added:"Black and purple carrots don't have as much beta carotene as the orange carrot, but are full of antioxidants.
"They are probably best very lightly cooked or raw to retain their colour."
Food expert Jane Milton also explained dark carrots' appeal. She said: "Darker-coloured fruits and vegetables are full of anthocyanin, which are powerful antioxidants."