Mechanistic explanations, aiming to disclose details of entities and their activities, employ the act of unpacking which, inherently and paradoxically, produces explanatory gaps--pieces of undisclosed, undetailed mechanistic information. These gaps, termed explanatory black boxes, are often perceived as counterproductive to the teaching of mechanisms, yet are integral to it, and their cognizant use is a nuanced skill. Amidst the discourse on mechanistic reasoning in science education, this paper focuses on biology teachers' perception of explanatory black boxes and the explicit discussion of them in their classroom. Using interviews with 11 experienced high-school biology teachers, we unraveled perceived affordances and constraints in teachers' use of black boxes in the context of challenges in teaching mechanisms. Utilizing the pedagogical content knowledge (PCK) framework, we expose a nuanced interplay of considerations related to strategies, students, curriculum alignment, assessment, and orientation toward science teaching. A constant tension existed--with considerations supporting and opposing the use of both unpacking and black boxing as teaching strategies--both within and between PCK components. In contrast, contemplating the explication of black boxes led teachers to illustrate this strategy as an intermediate one, attenuating constraints of both unpacking and black-boxing strategies while also promoting teachers' ability to align curricular items and endorse student agency. Implications for teacher training are discussed, emphasizing the need to make teachers aware of the involvement of black boxes in mechanistic reasoning, and familiarize them with black-box explication as an intermediate strategy that can enrich their pedagogy.