The odd thing is that on at least one occasion RJ had said the ratio was 10%, and here we are at 20%.
Perhaps with TG approaching and a resident forsakern, the shadow got better with recruiting, and it is far easier to do in such an isolated concentrated organization.

From the PoV's we see of several BA's (e.g. Sheriam, Elza), it seems that they were not all inherently evil. Elza seemed terrified when Semirhage tortured Rand - I don't think she had truly understood the evil nature of the "Chosen"... Many must have joined to gain stature and power, without fully understanding what they were signing up for. Once you are in, there is no way back but the path taken by Verin and Ingtar, and most people would never take it.

We actually saw some who chose the shadow not even for personal gain but out of the mistaken belief they could thus protect their loved ones, or perhaps out of desperation. That is certainly the case for Ingtar and, if I understood EoTW correctly - Kari Al'Thor. Some might have been forced by circumstances, like Verin. Some may even have been turned.

So just like there are very nasty non-darkfriends around (Elaida, Valda), there are darkfriends who were not inherently evil, at least not when they joined. Presumably they get more and more corrupt the longer they remain. This was what Ingtar saw in the Portal Stones in tGH.