Laser transmission welding relies on controlled laser–matter interaction at polymer interfaces, where the optical absorbance and transmission between parts to be welded governs the local energy deposition and the subsequent melting of both materials. This work explores an absorptance-based strategy to enhance laser–matter interaction independently of the intrinsic optical properties of the material that plays the role of absorbent component. A dark masking agent was introduced at the interface of a typical overlap joint to locally increase absorbance for a wavelength of 1070 nm (a continuous-wave ytterbium fiber laser). The influence of laser power on the fusion zone morphology was evaluated. Optical microscopy and mechanical testing were employed to assess the impact of the masking agent. Overall, it significantly altered the laser–matter interaction, leading to enhanced energy absorption and clearly promoting greater material melting. Microscopic observations revealed improved interfacial continuity and polymer cell bridging, indicating more efficient conversion of optical energy into effective load-bearing material. These results demonstrate that interfacial absorbance control is a powerful optical strategy for tailoring laser energy deposition in laser transmission welding. The proposed approach broadens the applicability of laser-based polymer joining and highlights the central role of optical absorbance in governing process efficiency and joint performance.