A genetic trait
The word "cordovan" in beekeeping terms takes it's name from a method of producing leather in Cordoba in Spain, the distinctive colour of which is variously described as reddish brown or purple.
Due to genetic influence, all the parts of a bee that are normally black take on this colour, which is quite noticeable. This pigmentation affects queens, drones and workers.
Cordovan bees are not a breed, as it is possible to have cordovan colouring in any bee, but it appears the vast majority are Italians, which is all that appears to be for sale when I looked. It seems the normal yellow colouring of Italians is lightened and extended, so they become what is described as "blonde" or "golden".
Cordovan bees have a reputation for being very docile and easy to spot queens. They are also considered to be very prolific, making huge colonies that are very hungry and are good at robbing, without being able to defend their own colony very well.
Cordovans don't appear to be much used outside the U.S. where it looks as if they are produced mainly for the hobbyist market, as I could not find reference to commercial beekeepers using them.
In writing the above I have gleaned material from a number of sources. I have no experience of cordovan bees.