Saint Blue Lance can transform into a Black Dragon, and after transforming, looks exactly like the Black Dragon in front of him. Austin said that Lance's ability to transform into a Black Dragon stems from mastering a secret technique called [Subjugation Seal].
To the Pureblood Dragons, the Subjugation Seal means if a Pureblood Dragon is told he resembles a Giant Dragon, he can transform into one.
This proposition clearly doesn't hold up.
Later, she and Austin researched it, and Austin said that Lance might be a Druid.
If he is indeed a Druid, it would somewhat make sense, but there are still many uncertainties.
She has fought with Lance in his Black Dragon form; his dragon body's strength is exactly the same as that of the Black Dragon.
That is a bit far-fetched.
They haven't thoroughly scrutinized the relationship between Lance and the Black Dragon; if they did investigate Lance's identity more closely, perhaps they might discover something.