Journalists Eric Laurent, 75, and Catherine Graciet, 48, are accused of demanding 2 million euros ($2 million) in 2015 in exchange for halting the publication of a book about the Moroccan royal family.
Both writers, who face up to five years in jail and 75,000 euros in fines if found guilty, have denied any wrongdoing and said they fell into a "trap".
The writers had already published a highly critical book on King Mohammed VI in 2012 titled "The Predator King", which was banned in Morocco.
Ahead of the planned publication of a second volume, Laurent in August 2015 met a lawyer representing the monarchy at the bar of a Parisian hotel, and warned him it contained potentially embarrassing revelations for the monarchy.
Morocco accuses the journalist of offering to halt the book's publication, originally due in early 2016, in exchange for 3 million euros. He allegedly later reduced that amount to 2 million following negotiations.
But Laurent says the lawyer representing the Moroccan royal family was the one who suggested a financial deal to prevent the information from getting out.
After the first meeting, Morocco filed a complaint and an investigation was opened in Paris.
This time under police surveillance, the lawyer and Laurent then met again at the same hotel later that same month.
They met a third time in late August 2015 at another hotel, where they were joined by co-author Graciet and both writers purportedly signed a deal to receive 2 million euros in exchange for not bringing out the book.
They were arrested afterwards in the possession of two envelopes each containing 40,000 euros in cash, which Morocco has claimed was the first instalment of the agreed larger sum.
During the investigation, both writers admitted to have agreed to a deal to halt the book's publication over geopolitical concerns, but denied the charge of blackmail.
Laurent's lawyer, Serge Portelli, has said both journalists fell into a "trap set up by the Moroccan services".
Graciet's lawyer, Eric Moutet, said she had "not taken part in any blackmail", and considered herself to have been "the victim of a trap".
After their arrest, it emerged that the Moroccan king's representative had secretly recorded all meetings, and passed on the recordings to investigators.
The recordings, which include many inaudible passages, are expected to be a topic of debate during the trial.