Blogs MU: Confussion with child theme css.

I am trying to change the color of the comment box of the Blogs MU theme to black. I know the following css code is where to make the change.

/blogs-mu/_inc/css/child-style.css

#comment-form {
background: none repeat scroll 0 0 #19252F;
border: 1px solid #17222B;
float: left;
margin: 0 0 20px;
padding: 3%;
width: 94%;
}

The question is which file on the child theme am I suppose to put this in? And how should it be entered?

Also, where would I go to change the text color? And which child theme should it be entered in?