I'd have said that, in the literary sense, a canon is established with hindsight and is thus subject to discussion and is certainly open to enormous subjectivity. Thus, whether any Black Library material counts as canonical depends on the standpoint of the person writing the list, and whether anyone else accepts that list depends on the strength of the argument put forward in defence of it.
Yes, this is postmodern and thus ultimately nihilistic, but it's how I was trained to think and I like it.
Army books might well be described as canonical. Different editions do take different approaches or even occasionally disagree with each other - but, just as an example, the four Biblical gospels are seen as canonical in the context of Christian literature and they disagree with each other. The only other thing I can think of is Dan Abnett - his 40K work has become a point of reference on all sorts of levels for other authors and is thus canonical. How often do references to Ravenor's "Spheres of Longing" crop up, for example? Or the Sabbat Worlds campaign?