The segregation kinetics of deuterated polystyrene (d-PS) within the grain boundary regions of a lamellar poly(styrene-b-isoprene) copolymer is analyzed using a combination of electron imaging and grain mapping as well as ultrasmall and small-angle neutron scattering. Solute segregation is limited to high-angle grain boundaries (that is, boundaries between grains having misorientations that exceed a threshold value). The accumulation of d-PS is found to be uniform across high-angle boundaries, reaching an equilibrium local concentration of solute within the boundary regions of about 52 vol %. This is interpreted as a consequence of d-PS segregation enabling the relaxation of perturbed chain conformations in the boundary regions, thus reducing the elastic strain energy that is stored in high-angle boundaries. The elastic strain energy is estimated using a continuum layer deformation model and compared to the experimental (relative) boundary tension that is determined by analysis of dihedral angles at grain boundary triple junctions. A McLean-type interface adsorption model is demonstrated to quantitatively capture both the kinetics and the extent of solute accumulation within boundary regions. The model reveals that the rate of segregation is sensitive to the mobility of the solute while the limiting concentration of solute within high-angle boundaries is determined by the energy of grain boundary defects. The results provide a basis for the interpretation of structure coarsening processes in block copolymer blend systems that are often found to result in more granular microstructures (featuring smaller grain sizes) as compared to the pristine block copolymer analogues and inform the development of processes for the strategic decoration of defects to enable new functionalities in block copolymer-based materials.