She met him again the next day at the same location, this time having organised the supply of a gram.
The undercover officer paid $800 for the drugs but when they were later analysed by forensic scientists, the substance was found to be bath salts.
Blair recently pleaded guilty to a charge of supplying methamphetamine and two charges of offering to supply, one of which was representative.
Once the undercover operation had snared the defendant, police obtained a warrant for access to the man's cellphone data.
They discovered that for three weeks, between March and April, Blair had offered the class-A drug on 14 occasions.
Defence counsel Anne Stevens stressed the quantities were always small.
In five of the instances, Blair had offered a ''puff'' for free, Judge Kevin Phillips said.
Mrs Stevens said her client was ''at the bottom of the food chain'' in his role as a street-level dealer, but police pitched the offending as more serious.
In written submissions before the court, the prosecution said Blair had clearly been comfortable enough selling to complete strangers, which indicated he was ''indiscriminate'' in his supply.
While the judge accepted the defendant was no kingpin, his role in the distribution was still vital.
''You may be at the bottom of the chain, but without people such as you prepared to sell to others, the persons above you would not have an income,'' he said.
Blair had been candid about why he got into selling the drug.
He told Probation he had become a regular P user and chose to fund his habit by selling too.
Mrs Stevens said he had used his time behind bars on remand productively.
Blair had been in touch with rehabilitation facility Moana House about therapy to address his dependency.
Judge Phillips commended him for the proactive steps he had taken and told him it was that which saved him from a prison sentence.
He sentenced Blair to nine months' home detention, to be served at his grandmother's house in Dunedin.