In this paper, we consider hyperspectral unmixing problems where the observed images are blurred during the acquisition process, \emph{e.g.} in microscopy and spectroscopy. We derive a joint observation and mixing model and show how it affects endmember identifiability within the geometrical unmixing framework. An analysis of the model reveals that nonnegative blurring results in a contraction of both the minimum-volume enclosing and maximum-volume enclosed simplex. We demonstrate this contraction property in the case of a spectrally-invariant point-spread function. The benefit of prior deconvolution on the accuracy of the restored sources and abundances is illustrated using simulated and real Raman spectroscopic data.