Multiphase biosourced nano-fibrillar polymer/polymer composites based on poly(butylene succinate-co-adipate)/poly(amide-11) blends (PBSA/PA11, 85/15 w/w) were successfully produced via a fast and simple melt extrusion process followed by tubular film blowing process. The effect of the take-up ratio on the morphological and rheological properties of resulted nano-fibrillar blown films was highlighted. The formation of stable nano-fibrillar structures is deeply discussed via rheological investigation to highlight the contribution of viscosity ratio, elasticity ratio and interfacial tension. SEM analyses indicated that increasing the take-up ratio during the film blowing leads to stretch the microfibrils and to generate fine nano-fibrils domains. Uniaxial extensional viscosity measurements showed a significant increase in the strain hardening intensity of the blends exhibiting nano-fibrillar structure compared to blends with nodular morphology. Maintaining the fibril morphology seems to stabilize the bubble during film blowing process without meanwhile sacrificing the mechanical properties of thin films obtained. The concept of in situ micro/nano-fibrillation is an efficient, cost-effective, and environmentally friendly technique for the enhancement of the processability and tensile properties of bio-based polymeric blown films and could find interesting applications in the design of novel bio-based polymer composites thin films for packaging industry.