Karasumi is known as one of the three great delicacies of Japan.

Karasumi is a food made from mullet roe that is salted, pressed, and dried.
Because they are expensive, substitutes are made from the ovaries of fish such as Spanish mackerel and cod, so the roe made from mullet ovaries is called "hon-karasumi."

So why is it called "karasumi"?

Karasumi was named after its shape, which resembled the ink called "karasumi" that was introduced from China. In the past, there was no refrigeration technology and it was stored at room temperature, so it turned black over time, and it is said that not only its shape but also its color resembled ink.
One theory is that when Toyotomi Hideyoshi, who was at Bizen Nagoya Castle, was asked what it was called when the local magistrate presented this food to him, he immediately replied, "It is Karasumi," and so the food was given the name "karasumi."

What are Japan's three great delicacies?

These are said to be karasumi (karasumi ink), sea cucumber intestines, and sea urchin.