If you have a login id for the blog, you can respond to a post.
When you are reading a blog post, scroll all the way to the bottom of the page and click on "LOG IN to leave a comment".
After you enter your login credentials, again scroll to the bottom of the page. Type over the words "write a comment" with your comments.
Once you start typing, you will see a + sign at the left. If you want to add a picture, click the + then click the camera icon that appears and select a .jpg or .png file from your device to upload. If you don't want to add a picture, ignore the +.
When you are done typing your comments, click the green Publish button.
Your comments go back to the poster, not to the entire blog membership.
An alternative to commenting via the blog is to send an email to firstname.lastname@example.org. The web gnomes will see that your email regarding the post gets to the right place.