To remove the sidebar area from all blog post, add this css code to Divi - Theme Options - General Panel - Custom Css

 
.single #left-area {
width: 100% !important;
padding-right: 0;
}
.single #main-content .container:before {
background: none;
}
 
To also include category and archive pages, add this code also:
 
.category #left-area {
width: 100% !important;
padding-right: 0;
}
.category #main-content .container:before {
background: none;
}
.archive #left-area {
width: 100% !important;
padding-right: 0;
}
.archive #main-content .container:before {
background: none;
}