Add background color

Adding background color or image to blogger blog comment form, can make your blogger page look more attractive and beautiful.
If you want your  blog comment box or form to have a background color this is what you do.
Navigate to your blogger blog "Edit Html" page by
Old Blogger Interface
Login to blogger --> Design --> Edit Html
New Blogger Interface
Login to blogger --> template from the drop down --> Edit Html
After any of the above interfaces just click on the "Expand widget" check button on the template page

find or search for
.post-outer .comments {
 
Now if you understand css just add your desirable background color within the .post-outer .comments curly braces or you can try this code bellow.
background-color:#eeeeee;
You can as well change the name of the color using either hexadecimal codes or by just writing the name of the color alphabetically.
 your code will now look something like this  

.post-outer .comments {background-color:color here;

Adding background image to your comment form

Upload image to blogger and get the image url
On the template page still under the same code i.e
.post-outer .comments {

add the background image script within the curly braces this way
.post-outer .comments {background-image:url('your image url here');
save the page and view any of your blog page.