Using photon correlation spectroscopy we show that isotopic labeling can strongly alter the pressure dependent thermodynamics of polydimethylsiloxane/polyethylmethylsiloxane blends, while each blend enhances its miscibility upon pressurization up to 2 kbar. The pressure dependence of the Flory interaction parameter, chi, changes by a factor of 4 when the deuteration of side groups is switched from one polymer to the other, but its pressure dependence is always negative. These results may be understood as being driven by the very different negative excess volumes of mixing for these different isotopic blends. We have attempted to unify our understanding of the role of pressure on blend thermodynamics into a single master plot by examining our data on volume changes on mixing vs the interaction parameter for the polysiloxane blends, and those from neutron scattering experiments on polyolefin blends and on a blend of a polyolefin and a polysiloxane. We find no universal trends when examined on this basis, and instead observe that variations in the chemical identity of the polymers in question and their chain lengths lead to very different plots. Further, since both neutron scattering and light scattering provide the same qualitative results, we argue that these results are not artefacts of either experimental technique. Our results strongly argue that the role of pressure on blend thermodynamics are much more complex than previously anticipated, and stress the need for improved theories for this important class of experiments.