To produce this technique Crona and Ragnarok must perform a Scream Resonance after absorbing a large amount of human souls. The combined power of performing this Scream Resonance after the consumption of so many souls allows Ragnarok to take the form of a dragon extending from Crona's back, complete with two large wings.
In this form, Crona's attacks are all enhanced, shown when they used Screech Alpha to destroy the Flying Dutchman's ship, Nidhogg, with just a single blow. They also gain flight capability, which is fast enough to give even Death the Kid a tough time keeping up on his Beelzebub. This form was only shown twice. Once during the Flying Dutchman incident and when Ragnarok and Crona brought the Mizune sisters to Death City. However this form has yet to appear again afterwards.
Crona's "Black Dragon" ability appears to be based on Emine's ability to create wings to fly, which is likely constructed out of his own blood, from Atsushi's earlier series B. Ichi.