A sexual relationship between an adult and a person under the age of consent, even if the event occurred long in the past, will be regarded as abuse and will probably be reported by any professional person except a Catholic / Orthodox priest acting within the confessional. It's one of the caveats always mentioned in counselling courses when talking about confidentiality issues. Failure to do so would be serious professional misconduct these days.

As far as non-professionals are concerned, although it isn't an actual offence to fail to report it, at some point if it is ongoing you might become liable to a charge of being an accessory; if they can fit you up for that, I have no doubt the cops would try. So given that, even Blackstone's contact with other BLs would probably leave a trail unless you are very careful, a person admitting to an ongoing relationship is putting his correspondent in a very difficult position.