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?
According to canadian lore, the original Het Red Axanthics were purchased by Corey Woods as very busy looking “Black Backs” and when bred together unexpectedly produced some weird Axanthic looking offspring, later becoming known as the Red Axanthics.
He also had another line of “black backs” that he called Black Stripes (they looked completely different from HRA, few or no alien heads and very clean like a Blade) that never produced Red Axanthics so not every genetic black back is Het Red. There are multiple different genes that can cause black back patterns.
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.