In the era of highly developed global value chains (GVCs), strengthened trade-investment nexus has made global carbon transfer more complicated, increasing the difficulty of emissions responsibility accounting and sharing. This study proposes an optimized shared responsibility-based accounting (OSBA) framework under global sequential production, which allocates responsibilities among producers, consumers, and investors, and then we compare the carbon emissions responsibilities of different economies and sectors under the OSBA frameworks and conventional accounting frameworks. Our findings show that under conventional accounting frameworks that inadequately consider carbon transfers induced by investment, trade, and sequential production, carbon emissions responsibilities of small developed economies with huge FDI outflows, as well as GVC downstream sectors, have been significantly underestimated. Behind these so-called “smaller emitters” in the conventional sense lie big black shadows (i.e., large volumes of carbon footprints in GVCs). Our study provides a new perspective on carbon emissions responsibility sharing along GVCs, highlighting the importance of integrating investors and global sequential production decisions into the responsibility-sharing system and emphasizing the role of multinational enterprises in greening global supply chains.