Well, Elaida would have been killed if she said no, the process is explained in the Companion. Ishmael started the BA when the Aes Sedai began to decline and faith/hope was at a low point. Once it started it was probably pretty easy to find the women who were most predisposed to becoming Black and "nurturing" them before bringing them in. I doubt many women actually sought the Black so much as were identified by the Black as good targets for recruitment. Certainly there were a handful who stumbled upon it, such as Verin, but only Alviarin and Liandrin seem to have been dark friends prior to entering the Tower.