His throat is blue simply because he had drunk poison, which erupted out of the sea throughout Samudra Manthan (churning of your ocean). Thus, he always wears a cobra close to his neck to mitigate the consequences on the poison. The title Neelkantha (Just one that has a blue throat) https://laxmi41737.ja-blog.com/29248717/not-known-facts-about-shiv