For more than five decades, X-ray timing and spectral observations have been used to study mass accreting stellar mass and supermassive black holes. The satellite borne Imaging X-ray Polarimetry Explorer (IXPE) and the balloon borne XL-Calibur experiment are two NASA led X-ray polarimetry missions scheduled for launch in 2021/2022. The two missions will provide new observables (i.e. the polarization fraction and polarization degree as a function of energy) that will allow us to conduct sensitive tests of the black hole accretion and emission models that have been developed based on timing and spectral information alone. For example, the new data will allow us to test and refine our methods for measuring black hole spins, and thus to assess which kinds of black holes nature makes. I will report here on KerrC - a new family of models that can be used to fit X-ray spectro-polarimetric observations of black holes, and show the results we can get from IXPE and XL-Calibur observations of accreting stellar mass black holes such as GRS 1915+105 and GRS 1739-278.