In the blog post content type's "Manage form display" section, go to the settings for the "Comment" field and select

"collect social profile information."

Create a custom comment type with theappropriate custom fields for collecting social profile links. Add a Comments field to the blog post content type, making sure it uses your new custom comment type.

Custom comment types per content type are not supported in Drupal 8

Add custom fieldsfor collecting social profile links to the default comment type. When configuring each field's visibility settings, go to "Content types" and make sure only the blog post content type is selected.