  The Blanco DECam Bulge Survey (BDBS) has imaged more than 200 square degrees
of the southern Galactic bulge, providing photometry in the ugrizy filters for
$\sim 250$ million unique stars. The presence of a strong foreground disk
population, along with complex reddening and extreme image crowding, has made
it difficult to constrain the presence of young and intermediate age stars in
the bulge population. We employed an accurate cross-match of BDBS with the
latest data release (EDR3) from the Gaia mission, matching more than 140
million sources with BDBS photometry and Gaia EDR3 photometry and astrometry.
We relied on Gaia EDR3 astrometry, without any photometric selection, to
produce clean BDBS bulge colour-magnitude diagrams (CMDs). Gaia parallaxes were
used to filter out bright foreground sources, and a Gaussian mixture model fit
to Galactic proper motions could identify stars kinematically consistent with
bulge membership. We applied this method to 127 different bulge fields of $1$
deg$^2$ each, with $|\ell| \leq 9.5^\circ$ and $-9.5^\circ \leq b \leq
-2.5^\circ$. The astrometric cleaning procedure removes the majority of blue
stars in each field, especially near the Galactic plane, where the ratio of
blue to red stars is $\lesssim 10\%$, increasing to values $\sim 20\%$ at
higher Galactic latitudes. We rule out the presence of a widespread population
of stars younger than 2 Gyr. The vast majority of blue stars brighter than the
turnoff belong to the foreground population, according to their measured
astrometry. We introduce the distance between the observed red giant branch
bump and the red clump as a simple age proxy for the dominant population in the
field, and we confirm the picture of a predominantly old bulge. Further work is
needed to apply the method to estimate ages to fields at higher latitudes, and
to model the complex morphology of the Galactic bulge.
