I'm reading Mackie, Blackburn and McDowell at the moment on secondary qualities and values, and having great difficulty understanding what their precise arguments are. Mackie seems to be saying that objective value doesn't exist - how does this relate to secondary qualities? Does comparing moral value and secondary qualities elucidate the nature and status of moral value?
Meta-ethics is not my cup of tea, but I remember reading those three guys' work in a seminar decades ago. This is my memory (impression) version of their work. There should be a lot of resources in SEP. The above three authors are important due to their places in meta-ethics.
Mackie argued that moral facts do not exist in the world, and thus impossible is moral knowledge. If you are seeing goodness or evil, rightness or wrongness in someone's action, then your perception is queer. Mackie thus is classified as a moral anti-realist.
McDowell conceded that moral facts do not exist in the world independently of our perception. They do however exist in the way secondary properties (Locke said things have primary and secondary qualities) exist. We humans can all perceive things in terms of their color, sound, or smell, even though these properties do not belong to objects. The ability to perceive these qualities is an anthropological fact. Likewise, we can perceive moral facts thanks to being wired anthropologically as such. If someone does not see moral wrongness when he sees kids are beating up a puppy, to McDowell, the person has a perception problem. McDowell thus is called a moral realist.
Blackburn called himself a quasi-realist since he thinks he can talk about moral facts and moral responses meaningfully despite holding moral anti-realism.