It is a genetically heritable trait.
You find and animal that has the “black back” phenotype and breed it. If some portion of the offspring also have the phenotype then it might be genetic. Repeat the breeding with the new generation and if you again get the phenotype you are probably safe assuming it is genetic
Do you know if snakes were sold as gentic black back prior to changing them to het reds?
Black back as a trait is common among the Black complex animals (e.g., BlkPastel, Cinny, HRA, Lori, Huffman, Gypsy, etc.,) but as @terces correctly points out, not every GBB is a HetRed (and, by converse, not every HetRed will have a black back).
GBB is, nowadays, used as a catch all for the many, and often unrelated, animals that produce the phenotype independent of an association with a known morph.