There are three basic ways to set up the optics for photography through a 'scope
1). Primary focus - the objective lens of the 'scope projects a real image directly on the camera's sensor (no ocular lens used, no camera lens used).
2) Projection ocular - A "projection ocular" is used - ie. an ocular that projects a real image on the camera's sensor is used (no camera lens used)
3) Afocal - a camera lens focused at infinity is used to look the 'scope's ocular (i.e the camera with lens replaces your eye.)
It sounds to me as if the "Barlow ocular" you are using isn't appropriate. Try simply removing it from the setup. If that fails (and it probably will because the objective lens' real image is inside the light tube) consider replacing the ocular with one appropriate to the task.
What's going on is that there must be a real image projected on the camera's sensor; this can be done with appropriate transfer optics.
You can see if an ocular projects a real image by holding it under a lamp & seeing if it projects an image of the light bulb; if a real image exists (like that cast by a magnifying glass) you'll see it.
Iowa Dave
See:
ScopeTronix: Digital Camera Imaging with a Telescope Tutorial
I've had good success using small, simple P&S cameras for Afocal work - such cameras have small lenses so vignetting is not too big a problem - just hold the camera over the ocular.