I really like the blue. Not for razors, but for knives and axes.
These days I use the yellow to remove metal around the edge, and then I use the blue to polish the edge. This gives me sharper edges than doing it the other way around. I think this has to do with the fact that the yellow has more garnets and thus dulls faster while freehanding (it is less forgiving).

I also have a solid block of blue, and that works fast and gives good edges. I think there is a lot of variation, but no necessarily correlating to the proximity to the yellow layer.

I remember Bart mentioning a couple of very special coticules that cost a ridiculous amount of money because there were only a couple of them ever found, due to the fact that they were just the result of a fluke of circumstances.