For the unfamiliar, Ashley Madison is a dating site that enables couples in monogamous relationships to find partners for illicit affairs. That's the baseline we're starting from. The company's new app, however, takes this concept one step further by providing users with disposable phone numbers that can be used for calling and texting without your spouse finding out.
The BlackBook app doesn't include access to the social network itself, so it's purely used for correspondence. The irony, of course, being that the very act of having an Ashley Madison-branded app on your phone is more suspicious to a scorned lover than any of the dozens of other disposable phone number services would be.