Elastomer blends are of high interest for tailoring materials with specific mechanical behaviour. However, while for the pure components the experimental characterization of the temperature dependent viscoelastic properties is usually well feasible, doing this for the blends is often difficult or impossible. One reason is that the blend components often have different glass transition temperatures which makes the resulting behaviour quite complex. In most cases when blending elastomers, heterogeneous morphologies are formed consisting of different regions with (nearly) pure components and finite interphases in between. Additional to the pure phases, especially these interphases influence the resulting viscoelastic properties significantly. For such cases, material modelling and numerical simulations can help to better understand the interactions between phases and interphases and to forecast the resulting viscoelastic properties. In this contribution we model and simulate an RVE of a binary blend consisting of natural rubber (NR) and styrene butadiene rubber (SBR). The modelling and the simulations are performed in the small strain regime, using linear viscoelasticity. A phase field variable is used to describe the blend morphology within the simulation. The blend morphology is based on microscopic images and the dependency on the field variable is derived from an energy formulation allowing sharp and diffuse interphases between the NR and SBR phases. Both, sharp and different diffuse interphases are numerically investigated and their influences on the mechanical behaviour are compared to elaborate experiments.