AZ blue are a standard zirconia belt, a very good price/quality balanced industrial belt.  They work well on hard metals.

The ceramics are well suited when you need maximum cut on very hard materials, they are well suited to knife making applications or when you need to remove very hard material faster. They also wear better than the blue zirconia.

We sill have a range of brown belts soon, brown belts are aluminum oxide belts and can used on all metals they are more suited to softer metals such as aluminum/brass copper and some steels. On hard metals they wont last or cut like a ceramic or blue zirconia belts. Brown belts do work fine on harder materials on the finer grits so for many applications 180 grit + they work well even on hard metals