I'm curious, I often heard that argument on the Internet after I researched that topic a bit further, but is that actually true?
Do you have a reference or source?
At least, I don't know of any authority who makes use of black helicopters (though I can't say it with certainty). It would be generally known. However, if so much people see them, there must be an explanation for it.
You find here a database where all(?) private German helicopters are registered: _http://www.helionline.net/home.html
There is also a nice gallery showing private and non-private helicopters.
For example, there are black private ones (which most likely are not operated by any ultra-secret MiBs):
_http://www.helionline.net/photogallery///36990//4.html
_http://www.helionline.net/photogallery///36987//5.html
_http://www.helionline.net/photogallery///37020//2.html
Bundesgrenzschutz?
_http://www.helionline.net/photogallery///36978//5.html
Do you live somewhere near the border? They look rather dark.
Helicopters used by Bundespolizei: _http://www.bundespolizei.de/DE/06Die-Bundespolizei/Ausstattung/Hubschrauber/hubschrauber_node.html
do helicopters need to have some sign on them in order for them to be allowed to fly in germany?
Yes, Flugkennzeichnung D-Hxxx. If it were totally unmarked, then it would be something out of the ordinary. But if it were ultra-secret government, why not simply driving a black Mercedes?? No one would care. Why would anyone who wishes to operate in absolute secrecy take such risks anyway (assuming human beings are the only party)? And testing technology? Well, they would rather do it somewhere in Nevada. They have Area51. They do not need the Alps.
Have you seen the helicopter from the left or right side? Standard authority helicopters are marked as such. Private ones have sign with letters only. "Next time" you should somehow manage to see it closely from the side in order to obtain a precise image.