I guess I should NEVER assume that people know how to do something. Here are he steps to posting a comment on a blog. Every blog is set up differently but the basic steps are all the same.
Ever blog is a bit different (color, layout, etc) but the link to leave a comment is almost always at the END of the post. On my blog this is how the link looks. The words PEOPLE CARED TO COMMENT is the link. Just roll your cursor over the words and click using your left mouse button.
Depending on how the blog is set up you will either see the whole screen change or a small comment window will pop up. If you see comments in the window you will need to scroll down to the bottom to find the open window to type your comment in.
You may see this "security" window pop-up. Basically, it just wants to know if you want all of the information to show up on your screen or not. If you say NO the screen will not look like the pictures below. I recommend clicking on Yes.
Type your comment in the window and then scroll down again. You'll be asked to enter an Anti-Spam verification code. Type it into the box underneath the string of 6-8 letters and numbers.
Continue to scroll down. If you are logged in to Blogger or Google you can use that ID as your identity. You can choose any of the four different ID types as your ID. If you aren't logged in to Google or you don't have a Google ID, on my blog you can select Anonymous as your identity. This option is not always available on all blogs.
Once you're done selecting your ID then you click Publish Your Comment and you're done.
You'll be given an opportunity to view your comment but essentially you're finished. You can go back to the blog and continue browsing.
Hope this helps.