I'm sure Blackburn isn't being altogether fair to Sam Kerstein (a friend and former professor) in this paper [.pdf], but I very much liked the overall gist of his argument. And I liked the conclusion, which puts me in mind of an ongoing conversation I've had with Julian over the last few years.
…the kinds of [rationalist] argument [against expressivism or sentimentalism] I have been discussing, are very deep-rooted. Partly, they represent a noble dream. They answer a wish that the knaves of the world can be not only confined and confounded, but refuted – refuted as well by standards that they have to acknowledge. Ideally, they will be shown to be in a state akin to self-contradiction. Kerstein acknowledges that Kant and neo-Kantians have not achieved anything like this result. But it is still, tantalizingly there as a goal or ideal, the Holy Grail of moral philosophy, and many suppose that all right-thinking people must join the pilgrimage to find it.
We sentimentalists do not like our good behaviour to be hostage to such a search. We don’t altogether approve of Holy Grails. We do not see the need for them. We are not quite on all fours with those who do. And we do not quite see why, even if by some secret alchemy a philosopher managed to glimpse one, it should ameliorate his behaviour, let alone that of other people. We think instead that human beings are ruled by passions, and the best we can do it to educate people so that the best passions are also the most forceful. We say of rationalistic moral philosophy what Hume says of abstract reasonings in general, that when we leave our closet, and engage in the common affairs of life, its conclusions seem to vanish, like the phantoms of the night on the appearance of the morning.