However, the issue for the jury would be whether or not it was Blair who made the sale.
The undercover officer gave evidence that the man who sold him the $20 tinny was approximately "6 foot to 6 foot, 1 inch" with white/grey hair tied in the rear and a full beard.
He said the man had "distinctive eyes" that were "quite big".
Straight after the transaction the officer returned to the police station and recorded his statement, including a description of the seller.
The next day he identified Blair from an eight-man photo montage as being the man who sold him the drugs.
In cross-examination Blair's lawyer, Fraser Wood, questioned the officer on apparent discrepancies between his written statement and the evidence given in court as well as the accuracy of his description.
Mr Wood said the officer's statement said the seller was approximately late 40s or early 50s, yet Blair was over 60.
He also pointed out that Blair was only "5 foot 11 inches" and questioned why, if the eyes were so prominent, they had not been mentioned in the officer's initial statement.
Referring to the police notes, Mr Wood said the officer had been at the door for a maximum of two minutes.
"I put it to you you were mistaken about this identification," he said.
"What you were actually doing was recognising Paul Blair from the photo you had previously been shown."
To both statements the officer responded: "No, that's incorrect."
The officer in charge of the operation, Detective Sergeant Ryan Yardley, gave evidence saying the briefing documents the officer saw before he went to the address had not included a photo of Blair and that Blair's name was not mentioned to the officer before he identified his photo.
Opening for the defence, Mr Wood said Blair was at the house that night but it was not him who sold the cannabis and he had no idea it was a tinny house.
"This is simply a case of mistaken identity," he said.
Blair said in evidence that when he arrived at his friend's house to cook her dinner there was a man there she introduced as her neighbour Andy.
He said it was Andy who went to the door when the officer knocked, while he was in the kitchen. He said he didn't see the visitor nor did he hear the conversation.
"I didn't know anything that was going on," Blair said.
He said Andy had a full beard similar to his but was about 10 years younger, a stone lighter and a bit taller than him.
The court had earlier heard how police had attempted to track down Andy by making inquiries with the occupier of the house and by searching police records but were unable to identify him.
Ms Tahana said Blair's evidence showed he had known the house occupier for about 10 years and had met her at her home and elsewhere often.
"You knew there was drug dealing taking place from that address didn't you?" Ms Tahana asked.
"No I didn't. I had no idea," he replied.
The trial continues.