We use molecular simulation to compare component dynamics of a poly(ethylene oxide) [PEO] and poly(methyl methacrylate) [PMMA] blend with that of a diblock copolymer of the same overall composition. The blend and the copolymer have different intermolecular packing, which leads to a difference in compositions defined over local length scales. These "effective concentrations" directly impact dynamic behavior through the chain connectivity model for blend dynamics, which is based on a single controlling length scale for dynamics equal to the Kuhn length. By comparing the change in dynamics expected on the basis of different effective concentrations with the actual change observed in the simulations, we find that this idea is quantitatively accurate for the PMMA component. For PEO, the controlling length scale for dynamics varies with the size of the observation volume.