Squid (and also octopus) belong to a group of animals called Cephalopods" and these animals most shoot out the ink. They store the ink in ink sacs between their gills. They jet it out with some water in the siphon, a part of their bodies that helps them breathe, move, and feed themselves. Octopus ink is usually black, but squid ink is mostly a blue-black color. They use the ink to help them escape from predators. If they feel under attack, they will shoot out the ink so the predator will not be able to see clearly and then the squids will swim away and escape the predator. Cool right?

Squid have an organ that contains ink inside of their bodies. They have another organ that squirt water (this is one of the ways that squid can move: they use jet propulsion). A squid can squirt ink from its ink-containing organ instead of water.

Great question! Squids and other cephalopods, such as octopi and cuttlefish, use ink to confuse their predators and thereby escape from them. The ink itself is contained in an ink sac, which is a muscular pouch that lies beneath the gut. When the squid wants to release ink, it contracts the muscles of the sac, which pushes a small amount of ink out of the opening out the "back side" of the squid.

The ink is mixed with a jet of water from the squid's funnel as well as mucus. The water helps to distribute the ink, and increasing the amount of mucus helps the dark cloud that is released retain a shape that looks similar to the squid. These pseudomorphs, or "false bodies" confuse the predator and allow the squids to escape!

In this review by Charles D. Derby here, Coleoidean produces ink while nautiloidean does not. Some nocturnal and deep-sea dwelling coleoideans has lost their ink sacs secondarily. The ability to produce ink is therefore evolved from a common ancestor.

I found a peer reviewed article that discusses this:

in this link.

Here is what I think the confusion is- all Coleoidea cephalopods have ink sacs and produce ink, but members of the subclass Nautiloidea cephalopods do not. From what I have seen, the primary type of cephalopod humans interact with are Coleoidea (squid, octopus, etc.) so it may be common to drop the first term and just call these ink producers “cephalopods” in general, even though there is another subclass that does not produce ink. I do think their ink is typically black, as this article says “Their ink, which is blackened by melanin, but also contains other constituents, has been used by humans in various ways for millennia.”

Far from "all" cephalopods shoot out ink, let alone black. Even if you limit yourself to squids, octopuses and sepias, there are more species that have no ink than species that do.

The taxonomic class Cephalopoda has two subclasses with living species, Nautiloidea and Coleoidea. Subclass Nautiloidea has six extant (existing/living) species; subclass Coleoidea has two subdivisions, Decapodiformes and Octopodiformes, each with several hundred living species. According to this review, members of Nautiloidea do not produce ink, but most of Coleoidea do produce ink. Accounting for the few species of Coleoidea which do not produce ink, one finds ~700 species of inking cephalopods compared to ~10 species of non-inking cephalopods. This article from the Smithsonian Institution also states that "Almost all cephalopods have an ink sac, a bladder that can suddenly release a plume of dense, black ink." Both of these are opposite the claim by our reader; if he has other information, he should present it so that we can determine who is correct and prevent further confusion.