This sounds like a manufacturing defect that went unnoticed during selective testing. Most often, uneven dye distribution. In extreme cases, could also be an impurity inside, an air pocket or nonflat substrate. There is not much you can do, short of returning the media for replacement by the retailer or the manufacturer, if it happens with every disc or most discs in the pack.
I've encountered a few problem spindles of Maxell-branded TYG01 and Fujifilm-branded TYG02 where an occasional disc (never more than about one in ten) would exhibit dark spots of varying shapes, but this happens with every manufacturer.