  The first black hole seeds, formed when the Universe was younger than 500
Myr, are recognized to play an important role for the growth of early (z ~ 7)
super-massive black holes. While progresses have been made in understanding
their formation and growth, their observational signatures remain largely
unexplored. As a result, no detection of such sources has been confirmed so
far. Supported by numerical simulations, we present a novel photometric method
to identify black hole seed candidates in deep multi-wavelength surveys. We
predict that these highly-obscured sources are characterized by a steep
spectrum in the infrared (1.6-4.5 micron), i.e. by very red colors. The method
selects the only 2 objects with a robust X-ray detection found in the
CANDELS/GOODS-S survey with a photometric redshift z > 6. Fitting their
infrared spectra only with a stellar component would require unrealistic star
formation rates (>2000 solar masses per year). To date, the selected objects
represent the most promising black hole seed candidates, possibly formed via
the direct collapse black hole scenario, with predicted mass >10^5 solar
masses. While this result is based on the best photometric observations of
high-z sources available to date, additional progress is expected from
spectroscopic and deeper X-ray data. Upcoming observatories, like the JWST,
will greatly expand the scope of this work.
