It's probaly some hard inclussion in the rock. In the old days, as today, the glued backing of a Coticule was never intended to be used as sharpening stone. Its function is to reinforce the Coticule. They used Blue stone, because that's what they had available in massive quantities anyway. Labor was cheap. Having employees sawing and lapping blue stones was the economic logical thing to do. Nowadays, labor is expensive. Vietnamese slate flooring tiles are much cheaper than having a Belgian laborer turn local blue rock into backing plates.
But the idea was the same. Not all blue rock is suitable for honing. In fact, there's only one blue vain that has ever been exploited as sharpening stone. That was one particular layer at the Ol'Preu deposit, known as La Veine Lorraine, aka "Salmon Rouge". It's a purplish BBW. But recent research at the university of Liège revealed that also other BBW layers at the Ol'preu deposit have some garnet content and can be used for honing. It also appears that blue stone situated adjacent to Coticule rock - in other words: the blue rock naturally joined to combo stones -, has sufficient abrasive properties to serve as a sharpening stone, although there are differences in speed between various specimen. Obviously, the blue backing of your vintage Coticule, has not been naturally adjacent to Coticule, otherwise it would have been a natural combo. We also don't know whether this stone comes from Ol' Preu. Likely not, because there were so many other mining locations that the odds are low.
While we're at it: in my opinion, you are far too quick to call a particular stone "La Petite Blanche" or "La Veinette" (there was another thread a while ago, but unfortunately I didn't find the time to address this issue then). Without knowing the exact mining location, is it nearly impossible to determine a layer. There were layers that are currently not being accessed by the last remaining mine. "L'allemande", comes to mind, but also "Les Petas", "Les Dados"; just to name a few. We don't even know if a Petite Blanche from Tier Du Mont, or from Old Rock, or from Regné, Hébronval or Ottré,which were all important mining spots, display the same morphological appearance as the ones we know from Ol'Preu.
But of course, all this information must never withhold you from enjoying that Coticule.
Sounds though, like to blue part won't be of much use, this time.
Kind regards,
Bart.