Francois is right. Blues grow better at around 1% salt, and start decreasing in growth rate around 4%. But, they will tolerate up to 10%. Beyond that, they are just about completely inhibited. When you surface salt, what you do is create a salt gradient, with the outer rind layer heavy in salt, and the middle getting progressively less. This difference is there for a while, many weeks, as the salt penetrates.
So to get a rindless blue, you salt the heck out of it. I mean, 10% is not a trifling amount. This prevents blue spores from germinating and there's an outer layer without mold, and the insides are still blue.