Blackman has joined Wycombe on loan until January 3.
Chairboys boss Gareth Ainsworth said: “For us to attract someone of Jamal's calibre to the club is a big coup for us, and is a result of the excellent relationship that we have with Chelsea both as a club, and me personally.
“He's a giant lad with massive potential to go far in the game and I'm sure he'll benefit from playing regular men's football and working under the guidance of Barry Richardson.
“Cameron (Dawson) was outstanding in his two games for us and we wanted to keep him here, but unfortunately we couldn't reach an agreement with Sheffield Wednesday to extend his loan.
“He's a great lad and we'll keep a close eye on his development as I'm sure he'll go on to have a very good career in the game.
“Jamal comes to us on a much better deal for the club and I'd like to thank Chelsea for allowing us to bring him in and trusting us with one of their young stars, who we are excited about working with and I'm sure he will learn a lot during his time here."