What cards does Barsad have that are against him that Sydney doesn’t know? (From Book 2 Chapters 6-10)

Barsard's cards contain secrets and a death sentence if discovered; they are black cards..... the cards of death.

Mr. Barsad saw losing cards in it that Sydney Carton knew nothing of. Thrown out of his honourable employment in England, through too much unsuccessful hard swearing there--not because he was not wanted there; our English reasons for vaunting our superiority to secrecy and spies are of very modern date--he knew that he had crossed the Channel, and accepted service in France: first, as a tempter and an eavesdropper among his own countrymen there: gradually, as a tempter and an eavesdropper among the natives. He knew that under the overthrown government he had been a spy upon Saint Antoine and Defarge's wine-shop; had received from the watchful police such heads of information concerning Doctor Manette's imprisonment, release, and history, as should serve him for an introduction to familiar conversation with the Defarges; and tried them on Madame Defarge, and had broken down with them signally. He always remembered with fear and trembling, that that terrible woman had knitted when he talked with her, and had looked ominously at him as her fingers moved. He had since seen her, in the Section of Saint Antoine, over and over again produce her knitted registers, and denounce people whose lives the guillotine then surely swallowed up. He knew, as every one employed as he was did, that he was never safe; that flight was impossible; that he was tied fast under the shadow of the axe; and that in spite of his utmost tergiversation and treachery in furtherance of the reigning terror, a word might bring it down upon him. Once denounced, and on such grave grounds as had just now been suggested to his mind, he foresaw that the dreadful woman of whose unrelenting character he had seen many proofs, would produce against him that fatal register, and would quash his last chance of life. Besides that all secret men are men soon terrified, here were surely cards enough of one black suit, to justify the holder in growing rather livid as he turned them over.