Black dragons have a lower chance of dropping it, but they have no ranged attack and can thus be attacked safely from a distance. The other monsters that drop it, such as the King Black Dragon, have a ranged dragon-fire attack and thus are more of a challenge to kill, but have a higher drop percentage. It just depends on what you would prefer (higher risk-higher reward, or lower risk-lower reward)
^He meant a sigil for a spirit shield. The only way to get a sigil is to kill the Corporeal Beast, which is very challenging and requires a large group of players, although can be done alone.
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See Draconic visage.