a black screenshot is an indication that there is no “real” desktop visible for the library at all.
(I would have been surprised if it had worked out of the box.)
I found an issue where someone hacked some lines into the imagehorizonlbrary to get XVFB working:
See also this:
(I had to use google translate )
Since I am planning a fork of the imagehorizonlibrary anyway (to finally include edge detection as a feature), XVFB support would be another interesting feature.
Try to see if this hack would work for you in general. We can then talk about whether this could be included in the new autogui-library.
)