How bad must a case of corneal abrasions be, when left untreated, for it to lead to blindness?

Not by itself. Corneal abrasion, even completly removing the outer lining will not lead to blindness even if untreated (although it will be painful for a while) Only if the abraded surface gets infected, or the abrasion is caused by an agent like lye or causes damage deeper than the surface, can vision be affected in the long run.