We propose a CubeSat with a wide field of view soft X-ray imager for high-energy and multi-messenger astrophysics. The BlackCAT CubeSat would use an X-ray Hybrid CMOS Detector (HCD) and a coded aperture mask to rapidly image a large region of the soft X-ray sky in the 0.2-20 keV band. The instrument would be the first of its kind utilizing an HCD and providing alerts and source localizations in the scientifically critical soft X-ray regime. It would detect and enable studies of gravitational wave X-ray counterparts, Gamma Ray Bursts (particularly at cosmological redshifts), and other X-ray transients and high emission states. The mission would thus function as a multi-wavelength and multi-messenger complement to several major present and future facilities including LIGO, VIRGO, IceCube, KM3NET, LSST, LOFAR, SKA, and CTA.