Aden Vanguard Captain Adolph:
The most important thing you need to gain the trust of the Blackbird clan members is to show them that you are not the enemy.
It's not enough that you are carrying the Blackbird Seal.
Defeat 60 of the Fortress Raiders and the Atelia Passionate Soldiers and 40 of the Fortress Guardian Captains.
After that, go find Glenkinchie in the 1st stronghold. He won't question your seal once you defeat those monsters.