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The accretion of matter onto black holes heats the surrounding materials to extremely high temperatures and drives outflows, producing a sudden and intense emission of light. Some of the most well-known examples include gamma-ray bursts, X-ray binaries (XRBs), and tidal disruption events (TDEs). In recent years, modern wide-field time domain sky surveys, such as the optical Zwicky Transient Facility (ZTF) and the Spektrum-Roentgen-Gamma (SRG) X-ray satellite, have opened up the discovery space of fast-evolving transients and enabled population analysis. In this thesis, I conducted a series of observational studies to understand the inner workings, environments, and demographics of high-energy transients powered by black holes. The first part of my thesis presents detailed studies on AT2019wey and AT2020mrf --- two transients discovered by SRG. First, I established that AT2019wey is a Galactic XRB with a low-mass companion star, and provided evidence that its central compact object is a black hole. Next, I demonstrated that AT2020mrf is a massive star explosion likely powered by fall-back accretion onto a newly formed black hole (with a rapidly spinning magnetar as an alternative power source). My work supports the idea that luminous fast blue optical transients form a rare class of engine-driven stellar explosions. The second part of my thesis concerns TDEs. I contributed to the discovery of two X-ray bright TDEs (AT2021ehb and AT2022cmc) and led comprehensive follow-up campaigns to track their long-term evolution. In both objects, using the NuSTAR and NICER X-ray telescopes, I identified novel TDE spectral features, which probe massive black hole accretion and jet launching. Additionally, using ZTF, I constructed the largest flux-limited sample of 33 TDEs, which enabled robust estimates of the optical TDE luminosity functions, host galaxy preference, and the black hole mass function. The emerging functional forms resulting from the large sample siz