By definition, emission from a black hole does not resemble that from an ensemble of normal stars. It has a different broadband spectral shape (the Spectral Energy Distribution or SED), a very high-luminosity density, and very different time variability properties. Roughly speaking, the broadband spectrum of an optically-selected AGN can be represented by a power law with roughly equal energy per decade from 1013 - 1020 Hz (Elvis et al. 1994). This is much broader than the ensemble of spectra from stars, which is roughly the sum of blackbodies with an effective temperature of 103 - 105 K. Superimposed on this power-law form are strong optical and UV lines from hydrogen, highly ionized C, N, and O, and a complex of low-ionization Fe lines. Thus, selecting sources on the basis of either their similarity to an AGN SED or their difference from a stellar SED is rather productive.
While the SED for optically-selected AGN is well studied, that for radio, infrared (IR), and X-ray-selected sources is much less well documented. Many of the radio-selected and hard X-ray-selected AGN show no indications of non-stellar colors in ground-based UV/optical/IR observations, and, thus, the use of the SED in these bands as an AGN indicator will not find the objects.