On the subject of "the Black Man"... there's some ambivalence on this one, I think. Though HPL has him as an avatar of Nyarlathotep, his actions here are rather stereotypical presentations of the normal role of the genuine "Black Man" of folklore -- especially concerning the New England witchcraft outbreak. So what we have here is Lovecraft putting a somewhat new spin on a very traditional trope in horror fiction; something he always felt very ambivalent about, as such traditional figures do tend to be hackneyed, predictable, and constrained by the long folkloric traditions assigned to them, whereas fresh, new creations which are given the appearance or feel of genuine folklore, allow the writer much more freedom and room for originality in theme and development. I'd agree that HPL makes him an impressive figure, but I'd say that's because he skillfully used those traditions to strike precisely those stereotyped reactions such figures still evoke when handled with an unusual degree of skill; not because there's anything particularly original in the figure or what he does with it. save in one or two minor incidentals.
As for Keziah, much the same thing can be said here; in truth, there's not a great deal of difference between her abilities and those exhibited by the sinister old beldame of The Lancashire Witches, in many ways. The main one is the hyperdimensional travel, and one can even see the vague precursor for that in Ainsworth's novel at times, in the witch's ability to evade capture for such a period.
And while gruesome in the extreme, Brown Jenkins' actions at the end are precisely the sort of gory scene that doesn't call for nearly as much skill; it relies much more on repulsion than genuine fear, and certainly more on a physical response than something resembling Lovecraft's eternal desiderata of "cosmic" fear or "adventurous expectancy". It relies on precisely that which he somewhat scornfully describes in Supernatural Horror in Literature as "the mundanely gruesome", rather than any frisson of spectral terror. However effective it may be, it does show a distinct falling-off from the effects he achieved in both earlier and later tales; and, again, Lovecraft himself was painfully aware of this, perhaps to an excessive degree.\
And I certainly didn't mean "simplicity" concerning his style here, but rather awkward use of phrasing, a tendency to redundancy rather than a modified reiteration, too-obvious striving for effect, and the like. In other words, a much less acute, precise, and exact use of the language to achieve many and particular emotional responses so common to much of his best work; a lessening of originality in concept and execution, including one of his trademarks, a seeming dichotomy which is actually an extremely acute awareness of the similarity of opposites (perhaps the best and most famous example of this is his "ecstatic fear" in "The Rats in the Walls").
So, while retaining a strong fondness for this piece, and certainly acknowledging the fact that it does often achieve some memorable heights, I still must maintain that it is among his weaker efforts overall... though given Lovecraft's abilities, that still means such things are often far above that achieved by so many others....
