Also, there are shellfish dyes, which are purple. They are quite interesting, because in Roman times there was a penalty of death if somebody outside the ruling elite was seen using these purple dyes. They had to use masses of these shellfish—hundreds of thousands of shellfish—to dye a relatively small piece of cloth.
When did people start synthesizing blue dye?
Up until about the middle of the 19th century all of the colorings were natural coloring. Then William Perkin [an English chemist] came along and made his mauve. That was a color you couldn't get naturally and it took the world by a storm. It was the first synthetic dyestuff to really take off.
The chemical constitution of indigo was worked out by Adolf von Baeyer in 1883. The first commercial synthesis was achieved in 1897 and was improved in 1901.
Do you think it's dangerous to eat blue food dye?
My own view is that any chemical should be regarded as potentially harmful when ingested. Note that naturally occurring chemicals can be just as harmful as synthetic ones—strychnine is no less poisonous because it occurs naturally!