One of Shiva’s characteristic features is His Blue Neck, so much so that acommon Shaivite* name is Neelakantha, Sanskrit for Blue (Neela) and Necked (Kantha). According to the Vishnu Purana, it started during the churning of the Cosmic Ocean, an event known as Samudra Manthan.
*[the Shaivas worship Shiva and His clan, as opposed to the Vaishnivas, or Vishnu devotees].
Now, as far as the Extended Hindu Universe goes, this is an epic crossover event.
The Devas (the Demigods) and the Asuras, their sworn enemies, were both at hand, as was Vishnu, Shiva and Brahma. Even their pets got in on the action: Shiva’s snake, Vasuki acted as a rope, while Garuda, Vishnu’s avian mount, flew the Mountain Mandara to churn the Cosmic Ocean (Vishnu took on the form of his second Avatar, the turtle Kurma, using his shell to prop up the Mountain).
As the churning process unfolded, numerous celestial beings and precious objects emerged from the ocean. Eventually, this would include Soma, the drink of eternal life, but not before the poison called Halahala surfaced. This toxic substance possessed the power to destroy all of existence.
As the Halahala spread, the gods and demons looked on in terror.In order to save the universe from annihilation, Shiva stepped forward and swallowed the deadly poison. However, even the God of Destruction could not survive the Cosmos-ending toxin.
To save Her consort, the Goddess Parvati intervened.
How, you ask?
She choked ‘im.
Yup, She Choked A Shiva.
She grabbed Shiva’s throat, preventing the poison from descending further. The poison stayed in His neck, turning it blue, resulting in the iconic image of the blue-throated deity known as “Neelakantha” or “Nilakantha”.
And so, if you look under his snake Vasuki (which He keeps coiled around His neck) you’ll find a deep blue spot – a deep blue spot that saved Everything, Everywhere (maybe All At Once).