You can change the background color of the comment box by:
1. Goto http://htmlcolorcodes.com/color-picker/ to select the color and copy the color code behind the # symbol.
2. Log in the blogger.com
3. Click Theme->Customize->Advanced->Add CSS (scroll down on 2nd column)->add the code below to the Add custom CSS box. (remember to replace colorcode with the code you copy in#1 above.)
#comment-editor {
background: #colorcode;
}
4. Click Apply to blog.