Well, given that someone got this intelligence to you, its entirely possible that the Shadow King is on your side. They honestly have no idea, but given that they haven't been detected, the Blackbirds almost certainly are fundamentally compromised. If the Shadow King does exist, and this isn't just all externally driven rather than internal corruption. The People are welcoming of refugees, and if a Highlands spymaster decided to take advantage of that behavior... it would be complicated and amazing that they haven't been detected up until now, but there's some level of conspiracy at work.