Half the time when a character in the movies or TV protests "that's blackmail," they're wrong. Cadsuane was troubled by her using their perilous situation (which was largely the fault of Rand, whom she probably saw as her charge by then) to induce them to do what she wants. It's close enough to blackmail for the analogy, which was drawn as a parallel to what she was doing with Ailil & Shalon. That part WAS blackmail, and she was blowing it off by recalling that she had essentially done the same thing to the three Asha'man.

Alanna would have been left in the dark anyway, since Cadsuane doesn't really trust her. In any event, Cadsuane could have blackmailed the Asha'man into going along with the others' requests to bond them, keeping the dirty part for herself, so the relationships between the sisters and Asha'man would not be tainted by the coersion. She tells Narishma that he had better go along if someone asks him if he wants to continue enjoying her protection, and Merise does ask him at Cadsuane's behest, maybe even not knowing that Cadsuane has taken steps to make him agree. That way Narishma doesn't resent her or see her as a captive, and will feel that way towards Cadsuane if anyone, rather than his bondholder.