I often use a 2B as my darkest (softest) lead, but I get it darker by layering it with harder ones, HB and 6 or 7H. I'll set the tone with 2B, go over it like lasagna with HB and 7H, and go back again the same way if I think it needs it.
See, this is where I get frustrated with my lack of knowledge. I've been doing the opposite. I start with a 2H or HB making the basic outline and shadows, then go in with my 2B and continue to layer. When I'm happy with how it's going, I'll spray a workable fixative on it and continue layering the darkest areas with my 2B, but I'm always afraid of getting a piece too dark to where I won't be able to lift out any mistakes.
My collection of pencils consist of HB, 2H, 2B, a soft charcoal and a med. Charcoal, so I don't have much of a variety to work with...but that will all change! The 2B is actually a .9 mechanical pencil and I've been meaning to get a .5 as well.
Thank you for the input toadflax! I'm definitely going to work on going darker. :)