@kitten It's a sensor one can touch to enable visually impaired beings to cross the road easier. There should be a button on the very bottom of that sensor thingy.
@kitten The regular button simply requests green light for pedestrians, the blue one is for blind and vision impaired people and enables accessibility features such as sound or haptic cues, or a longer (not earlier) green light.
Most cities in Germany integrated these accessibility features into the regular buttons, but a few towns (Augsburg I think off the top of my head) have them separate.