Yes but what was that concept of Rape? If you don't believe that the concept of rape has changed, in 2000 years, look at one example from fewer then 100 years ago, when it was considered rape if you tempted a women of good standing into having relations with you (women of Ill-standing were of course excluded). We certainly don't see that as rape, but in this country it once was, on the argument that the woman would have never entered into relations willingly if she hadn't been tempted in the first place.
The question is, did the Slaves see this as Rape, or a means of social advancement?