@KeshavSrinivasan add the answer

@SreeCharan But it's irrelevant to the question, which is about Indra's Vajra turning Shiva's throat blue.

@KeshavSrinivasan ok

@KeshavSrinivasan shiva's neck turned black after consuming poison. Isn't it?

@SreeCharan There are many different stories to as to how Shiva's throat became blue, perhaps due to Kalpa Bheda. There's the story that he consumed the Kalahuta poison, there's the story that sage Narayana choked him, there's the story that Indra struck him with a Vajra, and there's the story that Sukracharya created snakes which bit him. But yes, the poison story is the most famous.

@SreeCharan Take a look at this , it better and appropriate then last one although what he(D A Joseph) told was correct.