Since her "death" a few weeks ago, there has never been a truly definitive confirmation from anyone behind the scenes that Megan Boone was really never going to return to the show. The reality is that Liz's death was most likely due to Boone's real-life pregnancy and maternity leave. It's probably a safe bet that a reveal that she's actually alive will be the big cliff-hanger moment to end the season.
Given that, it seems unlikely we will find out in the finale how Liz is alive. Red (James Spader) is convinced she's dead, which means someone tricked him into believing such an idea. Was it Mr. Kaplan (who seems the most likely culprit) who faked her death? She seems to be the most likely culprit. Or was it a yet-to-be-introduced third party? We will hopefully find out more as Red takes down the man responsible for it all, Alexander Kirk, on the season finale.
The Blacklist airs tonight at 9/8c on NBC.