Armitage, 27, will serve a five-week ban after an RFU panel, chaired by Blackett, found the London Irish star guilty of a dangerous tackle on Bath wing Tom Biggs during the Aviva Premiership clash between the sides on October 29.
Armitage was also handed a three-week suspension for striking Bath lock Dave Attwood with a knee during the same fixture, a punishment he will serve concurrently with the ban for his tackle on Biggs. He will be available to return to action on December 9.
It is the latest in a series of disciplinary problems for the Trinidad-born full-back during the current calendar year.
He served an eight-week ban for pushing and verbally abusing an anti-doping official following a game against Bath on New Year's Day, and was then sidelined for another three weeks for a punch on Northampton fly-half Stephen Myler in April.
Armitage also missed England's World Cup quarter-final defeat to France after a high tackle on Scotland's Chris Paterson in their Pool B decider.
And Blackett, who said his punishment could have been more stringent were it not for Armitage's guilty plea and good conduct at the hearing, warned the player must heed the lessons of the various punishments he has faced this year.
"Delon has unfortunately had several disciplinary issues this year and it is a shame that he once again finds himself not playing. However, we hope that he will learn from these events and we look forward to him resuming the game he quite clearly feels very passionate about," he said in an RFU statement.
The RFU also announced that the dangerous tackle charges against Gloucester front-rowers Nick Wood and Darren Dawidiuk had been dismissed due to a procedural error by the governing body.
Prop Wood and hooker Dawidiuk were cited following tackles on Saracens' Brad Barritt and Owen Farrell during Gloucester's home defeat to the Aviva Premiership champions.
But the pair will face no penalty because their citings were handed down before the necessary consultation with the match officials, although they did receive a warning from the panel.
Farrell's citing for a dangerous tackle on Gloucester centre Henry Trinder during the same game was also dismissed, after the panel deemed there had been no act of foul play.
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