Detective Matthew Laurenson was in charge of the scene at Ms Blackbourn's home in the Auckland suburb of Glenfield.
Before he took the jurors through the police photographs, Justice Patricia Courtney gave the jurors a warning.
"There will be some very unpleasant images for you. Just take a moment to prepare yourself. It can be distressing to see these things.
Mr Laurenson said there had been severe fire damage in the master bedroom which had blown out both windows and caused the ceiling to collapse. There was little left behind of the bed and only the springs of the mattress could be seen.
He said police found the body of Ms Blackbourn, lying face-up, under the frame.
Nearby a set of keys were found with a name tag. The name on the keys read"Steve''.
Crown prosecutor Siobhan Buckley has previously told the court that the keys fitted the lock of Ellis' home in the nearby suburb of Northcote.
Mr Laurenson also responded to a jury question, passed to Justice Courtney, about the location of the knife.
"The knife was actually located further down, slightly nearer the bed.''
Defence lawyer Nicholas Taylor cross-examined Mr Laurenson and asked if he was sure the knife could not have been under the bed or on top of it when the fire started.
The detective referred to a photo in the police photo booklet. "You can see the spring mechanism which suggests to me that the knife was at least on top of the mattress.''
Pathologist Deborah Johnson said Ms Blackbourn's body was so badly burnt that in some places her bones were visible.
She said she found Ms Blackbourn had been stabbed twice and had bruising on her neck consistent with being strangled.
Dr Johnson said one of the stab wounds went into Ms Blackbourn's sternum and punctured an artery.
A second stab wound caught her just below the neck and caused significant bleeding that would have been fatal.
Dr Johnson said the wounds were consistent with a knife and would have been inflicted before the fire because there was little carbon monoxide found in her blood.
Under cross examination from defence lawyer Nicholas Taylor, Dr Johnson was asked if the cuts could have been caused by another sharp object, a nail or something else falling from the ceiling.
She said a nail would have made a round hole.
"I can't think of another object sharp enough to go through that far,'' Dr Johnson said.
She also said the angle of the wound was "completely the wrong angle'' to support a theory that it was caused by something falling from the ceiling.
