The assembly of lamella-forming block copolymer blend thin films on chemically nanopatterned striped surfaces was monitored in real time with small angle x-ray scattering (SAXS) in transmission mode. The strongest diffraction from the assembled grating structure was detected after 4.5 min of annealing as the temperature was ramped from 100 to 240 degrees C at a rate of about 20 degrees C/min. Real-space images were also obtained from samples annealed for specific times using top-down scanning electron microscopy (SEM) and this identified structures formed during annealing that are unique to the block copolymer blends. The data are compared to previously reported SEM and molecular simulation studies with pure block copolymers. Because it can be used in real time and probes the entire film thickness, transmission SAXS proved to be a useful tool for better understanding the block copolymer annealing process.