What is the ruling of eating squid’s ink?
Scholars have differing opinions regarding squid’s ink. Syafi’iyyah scholars ruled the black liquid excreted by some sea animals as najis. According to Syeikh Abd al-Rahman bin Muhammad Ba’lawi:
“The black liquid found in some sea animals which is not the meat or blood is ruled as najis. This is clearly emphasized in the book Tuhfah everything in the internal part of the animal, which isn’t part of its organs is ruled as najis. It includes this black liquid according to the reason explained. For it is blood or similar to blood.” 
The 5th Terengganu State Fatwa Committee which convened on 7th December 2011 agreed to retain the fatwa decision made by Allayarham Syed Yusuf bin Ali Az-Zawawi (Mufti in 1953-1975) which ruled squid’s ink as najis for the liquid is considered dirty and will smell horrible if left for a long time. This liquid is considered in the category of animal secretion and people with a normal appetite wouldn’t eat this liquid (ink). 
The difference of opinions on the matter is acknowledged by Kiai Haji Thoifur Ali Wafa from Indonesia:
“The black liquid found in some sea animals is an issue with the difference of opinions as to whether it is categorized as a liquid which is excreted from the internal organs which are najis, or not from the internal organs which is pure. An expert in the field should analyze this for it is closely related to what can be observed. I said: This black liquid, if it truly came from the internal organ, then it is similar to vomit. Hence, it is najis. Some of my teachers once said: This black liquid is something that is created by Allah for certain animals as a defence mechanism for it to hide from larger sea creatures. When there is a large sea animal that intends to eat it, it will then secrete this black liquid so that it can hide from the predator. Thus, this black liquid cannot be said to be like vomit or saliva, for this black liquid is its specific characteristic, which then ruled as pure. Wallahua’lam.” 
Squid’s ink or cephalopod ink is not an excrement liquid nor is it blood. It is secreted from the ink sac in its body but it is located outside its stomach, through a siphon tube before it is secreted out of its body to scare or distract its opponent. Thus, it is not najis.
The squid’s ink is black due to the melanin pigments, the same reason our hair, eye and skin have a dark or black colour. This liquid contains enzymes that produce melanin, peptidoglycan that is produced by sugar and protein, catecholamines which is a protein hormone, an amino acid that builds protein, metals such as cadmium, lead, copper which also help the production of melanin and toxins such as tetratoxin. 
There are no clear syarak evidences that prohibit its consumption, hence, the ruling returns to the original Islamic legal maxim which states:
الأصل في الأشياء الإباحة حتى يقوم الدليل على تحريمه
“The original ruling of something is its permissibility until there is evidence which prohibits it.”
The original ruling of eating sea animals is halal (permissible) as stated in the statement of Allah SWT:
أُحِلَّ لَكُمْ صَيْدُ الْبَحْرِ
“Lawful to you is game from the sea…” 
The Messenger PBUH said:
هُوَ الطَّهُورُ مَاؤُهُ، الحِلُّ مَيْتَتُهُ
“Its water is purifying and its dead (animals) are lawful (to eat).” 
Thus, without diminishing the respect towards the scholars who prohibit it, personally, I’m inclined towards the opinion that permits it. This is up to the person to follow one of these two opinions without saying the other is wrong. If one considers it as najis and it is unnatural to eat it, then he can simply not eat it. If he considers it pure and it is usually cooked in dishes, then he can eat it as he pleases. The ruling for this matter depends on the ‘uruf / custom of the place one is in as stated in a famous maxim العادة محكمة (the custom of a place determines the ruling of the matter). Wallahua’lam.