For a dark color like sea blue I'd skip the preshading. By the time you get a good coat of paint on the shading will be useless. I'd recomend post shading.
Take what I say with a grain of salt though. I'm not a big fan of preshading because while it adds interest to the paint, I don't think it looks right. I tried it a couple of times found it hard to control the end result. I much prefer the control of post shading. To each their own.
BTW, to your original question, I'd recomend black paint for preshading. No washes or anything that follows the lines. The result is to stark and will be hard to hide if you don't like it. The airbrushed soft edges of teh black paint blends into the paint better and dont' look to stark or bold, but gentle.