The dynamic heterogeneity of the segmental dynamics in miscible polymer blends has been alternatively interpreted in terms of either chain connectivity effects or thermal concentration fluctuations. Taking into account that both phenomena seem to be relevant, in this work we propose a minimal model which combines these two effects. This model basically assumes the self-concentration approach recently proposed by Lodge and McLeish [Macromolecules 2000, 33, 5278] and introduces a distribution of the effective concentration around a given segment mainly due to the effect of thermal concentration fluctuations. The proposed model has been checked in two blend systems, PVME/PS and P0CLS/PS700. Each of these systems allows to selectively observe by dielectric spectroscopy either the low- or high-glass transition temperature component of the blend, respectively. The model provides a good quantitative description of the dielectric segmental relaxation in both cases with only one free parameter: the variance of the distribution of the effective concentration. This parameter results to be hardly dependent on temperature, at least in the two blends investigated.