That might often be true, but "most" is definitely not "all". Certainly where I come from (Southeast Lancashire) the use of black sand in mortar was very common, especially in structures built from hard engineering ("Nori") bricks. Here is a cruel close-up from a typical example:-Am I missing something? In most brick structures the mortar lines would be a lighter colour than the bricks, rather than darker. Maybe this building had a dark colourant added to the mortar?
Despite appearances, it is not just a case of needing a re-point - the mortar actually is black. In models, the mortar colour seems to have a massively disproportional effect on how the finished brickwork turns out and personally, I think it merits careful consideration. That said, I have no idea what this prototype was originally like!!!