We investigate the influence of filler particles on the phase stability of a model blend of polystyrene (PS) and polybutadiene (PB). The upper critical solution temperature cloud point curve of a PS:PB blend is "destabilized" (upward shift of cloud point temperature) by the addition of untreated fumed silica filler particles. Preliminary cloud point measurements on surface-functionalized fumed silica particles were also performed. A small downward shift of the cloud point temperature was observed by functionalizing the filler particles with grafted polystyrene chains, while grafting with a silane coupling agent resulted in a large apparent "stabilization" effect or a decrease of the cloud point temperature. (Further measurements on surface-functionalized filler particles over a range of polymer compositions are required to determine the filler concentration dependence of the shift of the blend critical temperature.) Surface treatments were selected to demonstrate that modifying the filler particle surface chemistry can alter the blend phase boundary. These phase boundary shifts have significance in commercial blends containing dispersions of filler particles.