Let's begin by putting the most basic doubt regarding the difference between cold sore vs fever blister to final and eternal rest - they both mean the same thing! Both these terms are, and can be, used interchangeably to refer to the sores or blisters (technically speaking, lesions on the skin) that appear on either of the lips, usually the lower lip, after you've had a spell of common cold, flu, fever, or any illness that takes a toll on your immune system.