Coroner Marcus Elliott - who is presiding over an inquest into Blair's disappearance and examining four possible scenarios into Blair's disappearance: suicide, foul play, accident, or staged vanishing - is understood to be aware of the messages.
The family also say that Blair's Facebook profile was updated this week from "Kerry Blair" to "Remembering Kerry Blair".
They say they do not know Blair's log-in details or password, and that they have never had access to his account.
Blair was known to spend many hours online every day and had become "quite addicted" to a pirate internet game, the inquest has heard. He regularly spoke to his children through Facebook.The mysterious Facebook activity has upset the Blair family already desperate for answers into the mysterious case.
"We have no idea how to do this, let alone who did it," Dylan said.
During the inquest, Blair's children, Dylan Blair, Rochelle Foster and Cherise Blair speculated that three people, whose identity cannot be reported due to legal reasons, could have been involved in Blair's disappearance.
Blair's 9m craft was found drifting 200km off Taranaki nine days after he was reported missing.
But Blair was not on board and his body has never been found.
His family has raised concerns over the initial police response to the missing persons case.
Frustrated in the belief that police were spending more time investigating Blair's background and character than the seas and inlets, the family funded their own aerial search.
Rochelle told the inquest that she felt the police attitude at first was, "He's done a runner", and then when the boat was found, "Dad's committed suicide, let's work back from that and prove he has".
Blair's cellphone found on the boat showed a photograph taken at 7.35pm on the day he disappeared. It captures a setting sun glistening on the sea's western horizon, as well as the boat's side and rail.
For the family, the picture provokes more questions than answers.
"That photo is not a selfie," Rochelle said.
"Dad is not in the photo himself. That photo puts the phone on the boat. To me, that doesn't necessarily say Dad was on the boat ... I have a huge amount of questions if Dad was ever on the boat."
There's also a suspected missing shotgun.
"If you had a shotgun pointed at you, I'm sure you would do whatever the person holding the shotgun was demanding," Dylan said.
Police deny many of the family's claims.
Officers say they approached the "enormous" missing persons operation with a practical clarity that emotional and distraught family members could not.
They say there were no indications of foul play for them to investigate at the time, though they "kept an open mind".
Police officers have faced accusations during the inquest of falsifying theft claims against Kerry Blair days after his disappearance as a "means to an end" to get a telecommunications company to release his cellphone text data. A complaint has been lodged with the Independent Police Conduct Authority (IPCA).
The inquest, being heard in Christchurch, continues.
