Does this mean that Marx had no moral and ethical objections to capitalism? I think it's clear he did, protestations about "normative" versus "ethical" arguments notwithstanding. However, the objections themselves are a product of the social relations in which he was embedded; they were not the divine ethical pronouncements of some Oracle outside of/detached from history and social relations. Moreover, in practical terms, his moral objections had a social impact because they resonated with existing social relations (a string theory analogy comes to mind, but I'll let it go). Although moral philosophers prefer to base their arguments on what they perceive to be universal moral precepts, that's more or less an odd quirk of their particular culture; it is not the standard by which actual moral discussions in other social contexts should be assessed.
So did Marx make historically and socially contingent arguments based on moral assumptions? Yes. Did he claim that these moral assumptions float above history and social relations? Absolutely not.
Miles