Beth Matthews, 26, died a short time after taking the substance, which she told staff was protein powder, in March last year.
Matthews, originally from Cornwall, was described in court as “bright and vivacious”.
She was a well-known mental health blogger with thousands of followers.
Her inquest heard that she was being treated on a secure ward at the Priory, Cheadle Royal for a personality disorder.
The court was told that paramedics were called on March 21 after reports Matthews had taken an overdose.
Assistant Coroner Andrew Bridgman told the jury Matthews had “ingested a substance that came through the post, quite quickly became unwell, was taken urgently to hospital where she sadly died”.
In a statement, paramedic Kate Barnes said that when she arrived staff at the unit told her that Matthews “had a parcel delivered to the unit, which she opened in front of them and managed to consume”.
Inside the parcel was “a small plastic, screw top container”.
Matthews swallowed “an unknown amount” of the substance it contained and had apparently told staff the package contained “protein powder”, the inquest heard.
Barnes was told that patients were allowed to open their own parcels if supervised by staff.
The jury was told that the package had “foreign writing on it” and that the substance had apparently “been bought on the internet”.
A statement from Matthews’ mother Jane was also read to the court.
She said her daughter was “an incredible character” who was “bright and vivacious” and who “lit up the lives” of everyone she met.
She loved sport and excelled at sailing, completing the gruelling Fastnet race at the age of 15.
The jury heard that in 2019 after a suicide attempt Matthews suffered life-changing injuries.
She blogged about her recovery and her own mental health, gaining thousands of followers on Twitter.
Jane Matthews said her daughter had been able to help those who reached out to her and touched so many lives.
The inquest continues and is expected to finish next week.