Actually, it is essential to have a clear mental picture of what you want to draw before you draw it (assuming you're drawing from the imagination). If you're drawing a person, without reference, you need to be able to mentally picture how a muscle would look. The other day at work I was considering sketching something with IV fluids, but I didnt have the chance, nor did I have references. Without a clear mental picture and an idea of what I want to draw how can one be expected to successfully draw from the mind? To the starter of this thread: yes it is entirely normal to be able to look at a blank page and visualize what you want to draw. As I stated at the start of this reply, I also believe it to be essential if one isn't working with references.
The usual staples for anatomy: