The Blanco DECam Bulge Survey. I. The Survey Description and Early
  Results
  The Blanco Dark Energy Camera (DECam) Bulge survey is a Vera Rubin
Observatory (LSST) pathfinder imaging survey, spanning $\sim 200$ sq. deg. of
the Southern Galactic bulge, $-2^\circ <$b$< -13^\circ$ and $-11^\circ <$l$ <
+11^\circ$. We have employed the CTIO-4m telescope and the Dark Energy Camera
(DECam) to image a contiguous $\sim 200$ sq. deg. region of the relatively less
reddened Southern Galactic bulge, in SDSS $u$ + Pan-STARRS$grizy$. Optical
photometry with its large colour baseline will be used to investigate the age
and metallicity distributions of the major structures of the bulge. Included in
the survey footprint are 26 globular clusters imaged in all passbands. Over
much of the bulge, we have Gaia DR2 matching astrometry to $i\sim 18$, deep
enough to reach the faint end of the red clump. This paper provides the
background, scientific case, and description of the survey. We present an array
of new reddening-corrected colour-magnitude diagrams that span the extent of
Southern Galactic bulge. We argue that a population of massive stars in the
blue loop evolutionary phase, proposed to lie in the bulge, are instead at
$\sim 2$ kpc from the Sun and likely red clump giants in the old disk. A bright
red clump near $(l,b)=(+8^\circ,-4^\circ)$ may be a feature in the foreground
disk, or related to the long bar reported in earlier work. We also report the
first map of the blue horizontal branch population spanning the BDBS field of
regard, and our data does not confirm the reality of a number of proposed
globular clusters in the bulge.
