At their glass transition, simple liquids and polymers exhibit a broad spectrum of relaxation-with a width of more than 4 decades in time Blends of miscible polymers exhibit an even broader relaxation spectrum. Here, we assume that a blend can be considered as an ensemble of domains of various local glass transition temperatures. Using calorimetric data, with or without aging, we probe the distribution of glass transition temperature (T-g) of miscible polymer blends of polybutadiene (PB) and styrene butadiene rubber (SBR). Using the self consistent averaging method inspired by the Olroyd-Palierne model, we predict quantitatively, with no adjustable parameter, the viscoelastic spectrum of our blends from the T-g distribution obtained by calorimetry. This quantitative prediction confirms thus the assumption that mechanically, a blend can be considered as an ensemble of domains, each of which have a different glass transition temperature.