We develop a coarse-grained model to investigate the influence of nanoscale particles on the phase separation and the morphology of symmetric AB diblock copolymer melts. The microphase separation is modeled by the cell dynamical systems (CDS) equations, while the particle dynamics is described by a Langevin equation. We assume that the particles have a selective affinity to the A block and thus, can self-assemble and form clusters within A-domains. By varying the particle volume fraction, phi (p), we study the coupling between the microphase separation of the diblocks and the cluster formation for the particles. We also estimate the percolation threshold, phi*, for the particles and find that the presence of diblocks decreases phi* by a factor slightly greater than two relative to the case of particles in a homopolymer (phi* approximate to 9% in a diblock vs. 22% in a homopolymer). This result can be useful in designing new composites with increased electrical conductivity and/or mechanical strength.