My favourite dark nebula for the purpose of assessing sky conditions is the particular edge of the Coalsack which is seen when sweeping from the direction of the Jewel Box cluster(NGC 4755)
It is precisely because the contrast between the obscured side and the unobscured side is
not so great, that the level of sky transparency and sky darkness has such a great influence on what exactly you see in this part of the Coalsack.
In poor sky conditions, it is just possible to detect the influence of the dust on the obscured side......but all you see is a falloff in star density.
In reasonable dark sky conditions, it is possible to get some idea that there is actually obscuring material on the obscured side.
It usually takes excellent dark sky conditions for the obscured side to look really inky, in contrast to the unobscured side.
The Dark vs. Bright side contrast of a familiar dark nebula is a superb non-numerical test of how good the sky is.
Here is the section of the Coalsack I am talking about
(the cluster at top right is NGC 4755). Because the starry background is not super-bright and the obscuration is not high, you know you have an excellent sky when you can see this well in visual observations.
P.S.
As we discovered years ago when viewing through Mark Suchting's 8 inch binoculars, dark nebula observation really comes into its own with a binocular telescope. Some of us got so inspired by this that we sometimes spent most of the night viewing dark nebulae.
Contrast is a critical factor on these challenging dark objects; very good light transmission and very good baffling in your telescope are therefore important if you want to see dark nebulae at their best.
For my money, a good pair of apochromatic 5 inch binos would be the best instrument for dark nebulae and for general sweeping of the Milky Way.