Krishna explains to Arjuna that Rudra's throat became blue by snake poison.
When Rudra addressed himself for the destruction of the triple city belonging to the Asuras, the preceptor of the Asuras, viz., Usanas, provoked beyond endurance, tore a matted lock from his own head and hurled it at Rudra. From that matted lock of Usanas sprang many serpents. Those serpents began to bite Rudra, at which his throat became blue.
Popular story is linked to Samudra manthan. Interested to know if there are any other references explaining the blue colour to the events linked with Tripura?