Did you read the Shanti Parva chapter fully? Later on it gives the full story: "Driven off by an exclamation of Hun which Narayana uttered, the dart, its energy being lost, returned to Sankara's hands. At this, Rudra became highly angry and as the result thereof he rushed towards the Rishis Nara and Narayana, endued with the puissance of severe austerities. Narayana then seized the rushing Rudra with his hand by the throat. Seized by Narayana, the lord of the universe, Rudra's throat changed colour and became dark. From that time Rudra came to be called by the name of Sitikantha."