We investigate the effects of fullerene nanoparticles (f-NP) on the dewetting morphology in the immiscible temperature regime of blend films of polystyrene (PS) and polybutadiene (PB) on silicon substrate. As in our former work in the miscible temperature regime of this blend film, competitive partitioning of the f-NPs to the polymer-polymer and the substrate interfaces in blend films requires a larger concentration of f-NPs (similar to 10 mass %) to suppress film dewetting than in homopolymer components (similar to 2 mass %). In contrast, however, phase-separated blend films rapidly dewet into hemispherical droplets due to finite interfacial tension of internal blend components unlike irregular shape droplets obtained in miscible blend films. The effect of the f-NPs (1 mass % to 5 mass % f-NP) is to simultaneously reduce the size and contact angle of the dewet droplets, but the hemispherical shape of droplets is maintained, suggesting the f-NPs act to (a) only reduce the phase-separated blend interfacial tension but not fully compatibilize it into single phase and (b) reduce the blend substrate interfacial tension progressively. Selective solvent etching of the PS blend component reveals a spherical PS core enclosed within a circular PB shell at all f-NP concentrations. Confocal fluorescence microscopy reveals that the f-NPs are distributed in both phases consistent with the hemispherical shape and calculations that predict only a weak reduction of interfacial tension. The dewet blend droplet contact angle (likewise polymer-substrate interfacial tension) measured by atomic force microscopy shows a bimodal behavior, reducing rapidly (by 40%) at low (0.1 mass %) f-NP levels and significantly slowing down at higher f-NP concentrations. Molecular surfactant like behavior of the f-NPs in the blend films then provides an effective means of tuning dewetting blend film morphology dimensions without compromising phase behavior for potential applications in nanotechnology and nanomedicine.