  Binary black hole (BBH) mergers found by the LIGO and Virgo detectors are of
immense scientific interest to the astrophysics community, but are considered
unlikely to be sources of electromagnetic emission. To test whether they have
rapidly fading optical counterparts, we used the Dark Energy Camera to perform
an $i$-band search for the BBH merger GW170814, the first gravitational wave
detected by three interferometers. The 87-deg$^2$ localization region (at 90\%
confidence) centered in the Dark Energy Survey (DES) footprint enabled us to
image 86\% of the probable sky area to a depth of $i\sim 23$ mag and provide
the most comprehensive dataset to search for EM emission from BBH mergers. To
identify candidates, we perform difference imaging with our search images and
with templates from pre-existing DES images. The analysis strategy and
selection requirements were designed to remove supernovae and to identify
transients that decline in the first two epochs. We find two candidates, each
of which is spatially coincident with a star or a high-redshift galaxy in the
DES catalogs, and they are thus unlikely to be associated with GW170814. Our
search finds no candidates associated with GW170814, disfavoring rapidly
declining optical emission from BBH mergers brighter than $i\sim 23$ mag
($L_{\rm optical} \sim 5\times10^{41}$ erg/s) 1-2 days after coalescence. In
terms of GW sky map coverage, this is the most complete search for optical
counterparts to BBH mergers to date
