A Blend is a wonderful and much overlooked Illustrator feature that I've highlighted in my new book Creating Logos with Type. When you use this feature, Illustrator calculates what an additional object or objects would look like if you blended two others together...
Our obscure but useful Illustrator feature of the week is Outline Mode.
Outline mode can make it far easier to see what's happening in your artwork, because it only shows the bare bones of the shapes, as opposed to Preview mode, Illustrator's default way of showing things...