What is the difference between a single and double breasted blazer?

A single-breasted garment is a coat, jacket, vest, or similar item having one column of buttons and a narrow overlap of fabric. In contrast, a double-breasted coat has a wider overlap and two parallel rows of buttons.

The point of a double-breasted jacket is to accentuate the shoulders, which wide peaks accomplish better than anything else.

We always see peak lapels on a double-breasted jacket which is another reason that it's always considered a bit higher up on the formality scale.