Brush pen! The harder you push, the fatter the line gets. They're expensive though. I think I paid $3 or $4 dollars for the last one. If I have big areas of black, I will fill those in with a cheap chisel-tip Sharpie to conserve my expensive brush pen.
Also I found out it's a good idea to store your brush pens brush-tip-down so the ink stays down toward the brush. Otherwise the brush tip will dry out and there will still be ink up in the top of it where it doesn't do you any good.