Gabon Ebony will meet the criteria of hard, dense and black, but I have found that my experience with Gabon Ebony is that it is subject to cracking after the pen is built. I seem to have found this to be a problem with many of the very hard, dense woods, such as Snakewood, Ebony and Lignum Vitae.
If you can use acrylic, it will give you a very classy looking pen. Although, I have found that I didn't really get a perfect finish without using my Beall 3 wheel buffer. Solid black acylic will show even the smallest of scratches.
Good luck & be sure and post a pic of your finished product.
Bob