Increased focus on sustainability has made firms' value creation increasingly boundary spanning and complex. This begs the question of how business model research describes firm's external relationships, motivating us to undertake a systematic literature review. In analyzing 49 articles, we discuss and problematize four conceptualizations of external relationships - collaboration, alliances, networks, and ecosystems - arriving at nine propositions that clarify their meaning and refer to either firm-centric or collective business models for sustainability. In combining these findings with review results from conventional business model research we identify three main blind spots in extant business model for sustainability research, regarding coopetition, wider inter-organizational and sustainability tensions, and power. Based on these blind spots, we sketch a research agenda that could theorize business models for sustainability without neglecting their inherent tensions and contradictions.