That's the dye from Hexaplex Trunculus, but it's more of a bluish purple than a true blue. There seems to be modern ways to turn it into a true blue, but whethe

That's the dye from Hexaplex Trunculus, but it's more of a bluish purple than a true blue. There seems to be modern ways to turn it into a true blue, but whether the ancients had an unknown method is debated and unproven. It's worth noting that colour names in ancient times were a lot more blurry than now (the famous sea the colour of wine of Homer), and blue is still one of the most blurry.