An interesting article for sure. I have only read a handful of Blackwood's stories but Machen's thoughts on Blackwood's "The Man Whom the Trees Loved" do tease out something that bothered me about that story. I would also bring up Blackwood's novella The Damned which, while having some excellent parts, is pretty much an advertisement of the Theosophical Society with all the turn-of-the-century, proto-new age baggage that entails. That is, there is an effort to distill weird phenomena into a science, and the result is storytelling that is preachy and rather dated.
I guess the key difference between Machen and Blackwood is this: Both men were occultists, but Machen was at heart an orthodox Christian with a strong grounding in neoplatonism and other elements of traditional Western esoterism. In the vein, I suppose, of people like Marsilio Ficino or Jakob Boehme. I think this is true of a lot of the Golden Dawn associates. Blackwood was more attuned to new currents- theosophy, spiritism, etc.- which tried to merge some western occult elements, and what they knew of Vedanta, Buddhism, etc., with their interpretation of the ongoing scientific revolution. That is they were trying to create a new way that embodied ancient paths while also being completely "scientific," "rational", etc for modern man. Machen is much more ambivalent about the modern science- I don't know if he would fully reject it like some later "traditionalists" would, but he is certainly not interested in building his own worldview around it.