ThenDoctor said:
how do you know eisenhorn ever even went to the black ships he never said as much and im sure he would have at least mentioned it in the books
Actually, he does. Eisenhorn omnibus edition, page 42 (Xenos, chapter 3)
"I had carried one [subcutaneous ident marker] myself for a few years, as a child, before the Black Ships selected me and it was dug out."
(emphasis mine)