There were audible gasps when Powell announced his plea.
In the summary of facts, the court heard Powell turned up to the victim's house with a bat, and a large knife hidden in his trench coat.
Powell then admitted beating her over the head with the bat - until she was dead.
Blackmoore suffered 39 wounds to the head and neck.
In the summary, Powell claims he was to be paid $10,000 for killing her.
He will be sentenced on April 1.
Meanwhile, a trial date has been set for his co-accused.
The 47-year-old woman - who has temporary name suppression - has previously pleaded not guilty.
The seven-week trial will begin on February 15 next year.
Outside court, Anderson told Newstalk ZB he was "ecstatic" about the guilty plea because it had brought the case a year ahead.
"It's fantastic news that he's now grown a conscience and realised what he's done and he's standing up for what he's done."
He said the guilty plea was the "first step" with more to come.
Anderson thanked Detective Sergeant Todd Hamilton for his work on the case and said he did a "fantastic job".
"He's been on it from day dot and carried it through ... "