Short update: I have managed to schedule a hearing for a regular visa. The first available date was the 24th of August *cough*.
While this is clearly too late for Blackhat, but once you have a "regular" meeting scheduled you can ask to have an "urgent" meeting scheduled, too. Wether I am eligible will become clear when the embassy opens at 7am on monday morning.
The current plan is to call them and explain them why the entire thing might've gone haywire in the first place:
There's a special provision in the german tax code that allows for people with certain qualifications to act as special 'freelancers', essentially giving them a status very similar to one-person-companies ("Freiberufler"). It is not totally trivial to obtain this status - for example, you cannot simply be a 'Freiberuf'-programmer if you write "regular" software.
My agreement with Blackhat and all transactions were taxed in Germany under this status.
Personally, I think the fundamental issue in this tragic comedy is that the US doesn't really have such a special status for freelancers, and that therefore the US customs inspector did not understand that there is a distinction between a "regular Joe" and a "single-person company/Freiberufler". Hence the customs officer assumed that this entire thing must be some devious way to bypass getting an H1B visa for someone that would not normally qualified to get one. The frequent repetition of the question "why is your course not given by an American Citizen ?" points to something like that.
I hope that I can clear up this misunderstanding tomorrow morning, but right now, I am not terribly optimistic.