But as the 11pm deadline passed, there was no confirmation from either club that the paperwork had been submitted in time.
Campbell's future at Loftus Road has been the subject of speculation for some time after enjoying a successful spell on loan at Ipswich.
The 31-year-old scored 10 goals in a loan spell at Portman Road earlier this season and they were thought to be keen on bringing him back to the club.
Hull were then believed to be favourites to land Campbell after holding talks with the player earlier this week, before Blackburn stepped in.
Rovers boss Michael Appleton wanted to bolster his attacking options with Jordan Rhodes needing some goalscoring support.
But it remains to be seen whether Rovers beat the deadline and reports suggest they are in discussions with the Football League to ratify the transfer.