Methods and systems for real time, in situ monitoring and blending of hydrocarbon fluids from multiple transmission lines feeding into a downstream line or vessel are described. The method and system include the scanning of the NIR range on fluids within each of at least two transmission lines. The spectroscopic optical data from the two scans is used to determine flow rates of the fluids from each transmission line to, for example, achieve a desired energy content, physical properties, or speciation in the blended fluid.