In order to enable the comments for Blog posts you can check this article:
(Comments type: Telerik.Sitefinity.Blogs.Model.BlogPost).
Regarding the reply of comment, there is Feature request for this functionality in the Feedback
portal, where you can vote for it and subscribe receiving notifications on status change.
I hope this information helps.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items