The color black in turkeys was originally developed in Spain from turkeys taken from America. They were called Spanish blacks, and in England, they were called Norfolk blacks. After about 2 centuries they were brought back to America and mixed with the wild type. The name Spanish black has stuck and is still used today by many people.
That may well be true but today there are many Slate Blacks that are often sold as Spanish Blacks when many of them are not true blacks at all.