Brian Tung wrote:
If there are dark nebulae that are superimposed on already star-poor
regions, you probably wouldn't be able to see them unless you could
see very dim stars through the telescope, which would give a richer
background to discern the nebula. A smaller telescope wouldn't be able
to pick these out.

Since Barnard was working with 10" aperature astrophotography and the
catalogue lists nothing analogous to a magnitude (perhaps a contrast
index?), my current interest is which Barnard objects are visual and
which require astrophotography to bring out the contrast you describe.
The Tau-Per cloud is an example of region where an obscuring object is
visually detected by being an unusually star poor region.

- Canopus56