Have you ever heard of the Lewton Walk, in which a character walks through a night environment turned strangely threatening? Years before Val Lewton, Hitchcock takes a guilt-stricken Alice on such a walk through the deserted streets of London.
No, I hadn't heard about that! But now that you mention it, yes, I can vividly recall walking scenes in "The Seventh Victim" and "Cat People" that do have heightened tension in them. I'll have to seek out for more Lewton films! Thanks Prof!
I don't have the book at hand right now, but I don't remember Hitchcock specifically talking about this subject to Truffaut. The use of shadows by Hitchcock in this and in many others of his movies was obviously inspired in his stylistic mentors: "The Germans. The Germans." Donald Calthrop's shadow growing on the building front right after the killing reminds (me, at least) of Murnau's "Nosferatu" and Max Schreck's shadow climbing the stairs to meet Ellen Hutter. However, when he talked to Truffaut about "Blackmail", Hitchcock mentioned the Germans only once and indirectly, about the use he made of the Schüftan process to film the British Museum climax.
Hitchcock did mention a little bit about the shadows in Blackmail to Truffaut:
"I did a funny thing in that scene, a sort of farewell to silent pictures. On the silent screen the villain was generally a man with a mustache. Well, my villain was clean-shaven, but an ironwork chandelier in his studio cast a shadow on his upper lip that suggested an absolutely fierce-looking mustache!"