As you know if you get into a car on a hot sunny day with dark seats instead of light colours, black releases and absorbs radiation (radar) better. This was more important in the older days of radar. New liveries take into account that this `helper' is no longer necessary.
Yes, the BA 757 black ring signified a trial with a Microwave Landing System.
` Yeaah! Baade 152! Trabi of the Sky! '