Neelakandan is one form of shiva.
Legend
Once upon a time devars and asura were trying Samuthara mandam in parkadal. That time Allakala visam came first. It is a cruel poison. To save the world shiva ate the poison. Parvathi suddenly picked the neck of shiva. So poison stay shivas neck. Shiva's necks color changed to blue. This form of shiva is called Neelakandan.
In Tamil Neelam means blue color and Kandam means Neck.
References
Neelakandan Wikipedia(Text) CC BY-SA