In looking for a bit more information on visual observation of dark nebulae, I've come across a great list for this very purpose:
Best of Barnard's Dark Nebulae
A very helpful note on the list is the grading of the 'darkness', which is on a scale of 1 to 6, with 6 being the darkest against its parent background.
RobF has posted a stunning image of the dark nebula
B84, "the Claw", in Sagittarius. This one is rated as a 5, so should be a very nice one to chase down. Within the same field of view also lies a globular cluster and a planetary nebula. If the PN is difficult to spot from amongst the background stars, "blinking" it with an OIII filter will help here. If you are unfamiliar with "blinking", have a look in the "honing your observing skill" sticky, where I posted a very easy to make 'blinking paddle' for this purpose.
Mental.