Problem:
I have created a SharePoint site using "Team Site" template and after that I want to use blog functionality in Same site.
Solution:
You just need to activate a Blog feature in you existing site.
Enable-SPFeature -identity "BlogContent" -url <<the url of the site>>
Now you are able to use Blog feature in your existing site
go to Page >> Edit Page >>Add Webpart button from ribbon >> Apps >> Posts