For Black, a rhetorical transaction is a complex and a process of three elements of rhetoric: strategies, situations, and effects. These elements are understood as the following [1, p. 133-135]:
- Rhetorical Strategies: the characteristics of the discourse
- Rhetorical Situations: extralinguistic influences on the audience
- Audience Effects: responses to the strategies in the situations
Taken together, these three elements converge to be a complex process (or phenomenon) that can be articulated to describe the effect of an artifact (or deliverable) which he names the rhetorical transaction. Certainly, one could approach TC through this heuristic in most genres including document design, proposals, and web design.
For instance, what are the characteristics of a given deliverable? This sounds like a genre to me. A proposal should contain certain elements while a resume would include different elements. All technical communicators should thoughtfully consider the conventional elements in light of the situation and audience effects. Also, "extralinguistic" influences sounds, to me, close to document design and other strategies of visual rhetoric while Usability would address the audience responses. All three work together to adequately describe the all important context of any deliverable. How would our service-level TC classes be enhanced if we, as instructors, thought about teaching writing through the lens of Black's Rhetorical Transaction?
Of course this is not a comprehensive treatment of Black's method but simply the initial musing for me on this matter. Black's subsequent writing on "Exhortation" and "Argument" may also offer good strategies for TC. I am simply wondering at this point. Hmm.. here is another dissertation topic.
 Black, E. (1978). Rhetorical Criticism: A Study in Method. Madison, WI: The University of Wisconsin Press.