The True Black Dragon (真の黒龍, Shin no Kokuryū) first appears after Ragnarok absorbs all the souls collected by Ghost Ship Nidhogg. To produce this technique Crona and Ragnarok must perform a Screech Resonance after absorbing a large amount of human souls. The combined power of performing this Screech 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.