A black bull stands in the dark and does not blink. The aura is not given to it. You build it with your own grip. Hold the floor, feel the rumble climb, and when it caps, let go. The bull charges. The aura is yours to keep.
Press and hold the bull. The longer you hold, the higher the aura climbs. Let go at full and the bull charges, and your aura banks to the ledger below.
Every other coin tells you the aura is already inside the token. Buy it, hold it, wait. Nothing arrives.
The black bull works differently. It stands in the chamber and stares at the floor until somebody puts their weight on it. That somebody is you. Hold long enough and the rumble turns into a charge you can feel in the screen.
So far the bull has charged 0 times tonight, and not once for free. The aura you see in the ledger is aura you earned. Keep your hand on the floor.