I do think this is a consensual relationship. Of course Billy could say no. Even when there is a power hierarchy, there can be consent. Otherwise there would never be a consensual relationship possible between people from different classes or ages. This is not romantic love, but in the canon story Holmes clearly cares for Billy's well-being (he rushes into the room when he sees Billy threatened), and Billy obviously knows Holmes well and enjoys taking care of him. There is no reason for Billy to blackmail Holmes. And yes, Holmes is an intelligent man, but in this incarnation he has physical needs, too. And like for many gay men of his age, this kind of arrangement would work well. My two cents only, of course.