Maybe she was an ‘out of work prostitute’ OR maybe just a very unpopular one. If she didn't have many customers she might remember who the lucky guy was. Or she could have been a long time mistress, one who threatened to black mail him if something wasn't done about the baby. Not that I believe this BS but that is a more reasonable explanation than if he got some one time hooker pregnant.
If there was a black mistress she must be either dead or so well paid off that she never considered coming forth with this news. “My son is the president of the USA. Bow down ya’ll” LOL!