How to get comment thread of a blog post using REST API?

To list the comments for a blog post, the REST Comments List Endpoint can be used.  The blog post's ContentId would be passed in as the ContentId for the call.

Thanks Mathew, I'm able to get comments for blog post using blog ContentId.
Now I have 2 blog posts in a group but why is it that I'm getting same set of comments for 2 different blog ContentId?

The ContentId should be for the blog post, so each blog post should be using a different ContentId.  Not sure from what you said above if you are doing that.