The Lions have won just one of their last six league games, while Blackburn's last victory in any competition came with their unexpected 1-0 win at Arsenal in the last round.
Michael Appleton's men have lost four and drawn one of their last five since that win at the Emirates.
Millwall dispatched Luton Town 3-0 in round five. This tie will be the first FA Cup quarter-final to not feature a Premier League side since 2008.
Possible starting lineups
Millwall: Forde; Shittu, Wright, Smith, Lowry; Afobe, Beevers, Woolford, Dunne; Marquis, Keogh
Blackburn: Kean; Henely, Dann, Hanley, Lowe; Pedersen, Bentley, Olsson, Jones; Campbell, Rhodes
Sports Mole says: 1-0