Silver do nor reacts with HCl but in the experiment not only pure HCl is present.
Change on silver surface after long treatment with HCl can be caused by traces of sulfur, hydrogen sulfide, oxygen, nitrogen, nitrogen oxides - all can be present during experiment.
I think the fastest (though very slow indeed) can be surface reaction of HCl with Ag in the presence of oxygen. Then a small amount of AgCl of insoluble can be formed, AgCl decomposes to Ag (grey blue) and radicals of Cl under light.