About the link you provided
The black bar is part of the .pagewrapper but there is no width specified for pagewrapper.
.pagewrapper {
left: 0;
position: relative;
transition: left 500ms ease-in-out 0s;
}
The post content is set a max-width: 700px, it will not extend the boundary.
.wrapper {
margin: 0 auto;
max-width: 700px;
width: 90%;
}
The black bar is entirely new section which has this code, no width set for it:
.postcontents.dark {
background: none repeat scroll 0 0 #2c2e32;
color: #e2e2e2 !important;
padding: 90px 0;
}
There is a CSS specific solution.
If you need to use the block quote which is 100% of the width, you need to make it come out of the normal flow of the #container.
.blockquote {
position: absolute;
width: 100%;
left: 0;
}
Blockquote within container
Blockquote outside the flow
A better solution will be to create a custom class called blockquote-full-width and allow the users to use it.