The South Yorkshire club currently sit fourth, just one point and place ahead of their hosts as they face their penultimate game of the regular Championship season.
If Rotherham win, score four tries and deny Nottingham any bonus points, they will have secured a play-off berth for a second successive season.
But anything less will see them head into the last day still uncertain.
Nottingham beat them 28-19 at Clifton Lane in January and Blackett said: “This game is going to be a massive test for us.
“We know just how good Nottingham are after what happened earlier this year.
“They’ve got very good players throughout and they were unlucky last week to narrowly lose away to London Scottish.
“I think the game will be very close and it’ll come down to those little moments at crucial times.
“We’ll need to execute everything as well as we can.”
Tom Barrett comes in at fly-half to replace James McKinney (knee) and captain Tom Cruse returns at hooker.