[Mithadan]

I am not at all familiar with Blackwood or his work. From his biography, which describes him as a well-known English broadcasting narrator, journalist, novelist, and a CBE to boot, I see no reason to doubt that Tolkien was familiar with him.

I would point out that the supernatural nature of old and dark forests is well-established in both folk tales and myth. Consider the Black Forest and its reputation, the wood of the suicides in Dante, the role of trees in druidic lore.

Characterizing willows as evil may be a bit odd. From some quick research, willows are typically associated with healing. From personal experience, groves of willows are a source of constant motion and sound. The motion and sound are, indeed, relaxing and could easily induce drowsiness. Old Man Willow may be the proverbial "bad seed."