It is known that for any finitely generated group G from the large class of "locally graded" groups, satisfaction of an Engel or positive law forces G to be virtually nilpotent. In [2] Sarah Black gives a sufficient condition for an arbitrary 2-variable law w(x, y) ≡ 1 to imply virtual nilpotence -though only for finitely generated residually finite groups. We show how the Dichotomy Theorem from [4] for arbitrary words w(x 1 , . . . , x n ), encompasses Black's condition, extending it to the nvariable case and a certain large class S (however still falling short of the class of locally graded groups). We infer in particular that her condition is also necessary. We also deduce a simplified version of an algorithm of Qianlu Li [8,9] for deciding whether or not a given law w(x 1 , . . . , x n ) ≡ 1 satisfies the extended version of Black's criterion.