Type 3 variation on 1959 Topps 6 Alex Grammas. There is a large black smudge in the lower left background and reaching into the bottom border, just touching the lower right corner of the triangular Cardinals logo. The smudge can be found in a dark black but also a more muted black. These are evidently the same variation in different states of intensity. This variation occurs on approximately 6% of examples according to a survey of cards.
The dark black variation looks unseemly and like an ink blotch, so it likely discourages people from submitting them for grading. It is, however, a fully natural result of the production process. This variation occurs on approximately 40% of examples showing either the dark or muted spot, 2% overall, according to a survey of cards.
The much less common muted black variation is not as distinct. This variation occurs on approximately 60% of examples showing either the dark or muted spot, 4% overall, according to a survey of cards.
Just for fun, here is the strong black mark variation autographed.