Is epistemic supervenience a conceptual truth?

This is what is needed in order to avoid Blackburn's critique. But epistemic supervenience has been challenged by Keith Lehrer. Cuneo notes, in a footnote, that if Lehrer is right about epistemic supervenience then the ethical doesn't supervene on the non-ethical either. But I thought supervenience was supposed to be a conceptual truth, in which case Keith Lehrer is making a conceptual error. If we think that he's not making a conceptual error, then we have to ask whether it's a conceptual truth that ethical truths supervene on the non-ethical (as has been assumed).

It's his coherence theory of justification that allows him to ask whether epistemic supervenience is false (let alone a conceptual truth!), and if we think that this would be plausible in ethics maybe that would answer Blackburn's claim that it's a conceptual truth. (But here too we have to be concerned about whether, in a coherence theory of ethical justificaiton the work is being done by ethics or by a coherence theory of justification, in which case we might just be forgetting why ethical supervenience makes sense).