I'm not sure about the blue motoro, I know that it is correct that they lose their blue coloring but I do not know how long it takes. There is no such thing as a black motoro what you are probably referring to is a dark varient motoro.
blue fades as they grow.... search posts made by miles he posted alot about how its the disk being very thin.... also black motoro is just very dark peruvian motoro....
well considering they are all the same and blue motoro is supposed to be real skinny disk.... dark motoro or reg motoro what ever you like.... and me personally motoro wise only really like the columbian marbleds....