That's a lot of deathpit.
Honestly, art tends to be better when maps are done art-first. The problem is that it makes the gameplay parts much harder.
You also would have to make your dev blocks shaped in a way that you can create the details you want. Without anything that lays focus on it your map is going to be quite dull detail wise. Still, as mapper you usualy already have such idea of an area and make your brushwork to match it.Nope, you have to learn how to turn dev blocks to real details then.