(Going by the fact that these are used in packaging.) These are black lines or black marks on the back or backing of a roll of labels that allows the printer to determine where one label stops and the next one starts.

From the lexicon link https://www.bobines-papier-thermique.com/infos/lexique

"SPOT NOIR : impression noire (d’un pavé ou d’une ligne), généralement au verso (sur le backing), permettant la détection des étiquettes par l’imprimante. Indispensable pour les étiquettes transparentes, ou les étiquettes non adhésives avec des formes particulières. Synonyme : Marque noire, Black mark."

From Citizen printer manual https://www.citizen-systems.co.jp/support/download/printer/m...

"Using continuous media and label media without gaps between labels
Set the media so that a section without a black line (or label face stock for label media) does cover the platen roller and media sensor."

From barcode-labels.com https://barcode-labels.com/technical-support/label-lingo-lab...

"Black Mark – Some label printers may also have a “reflective” sensor that relies on black lines or marks printed on the back of the label liner. The width of the black mark is synchronized with the label size to tell the printer when to print label to label. Black marks are particularly useful when labels or tags are “butt cut” and separated only by a perforation or face cut. "