He has taken up the option of returning to a full-time environment with the South Yorkshire club having now switched from part-time status.
Blackmore, 22, made his Super League debut for hometown Castleford in 2012 before moving to Huddersfield Giants for a couple of seasons and playing a handful of games there.
He joined Featherstone in 2014 and was a regular last season scoring 15 tries in 23 games.
Blackmore will compete with the likes of Rob Worrincy, fellow new signing Gary Lo and youngster Ryan Millar as the Eagles continue their close season recruitment drive.
“Like many of the players we have brought to Sheffield Eagles during the close season, Ben is another young and exciting prospect,” said director of rugby Mark Aston.
“He fits the profile of the players we are wanting to work with, and like many who have come through the door, he arrives with a good background and plenty of potential.
“His ability isn’t in doubt because he was given an opportunity at Super League level. He has a good grounding and knows what it is like to succeed at Championship level too.
“He has good experience for a player of his age and knows what it takes to succeed in this division.”