  Davies et al. (2019) established that for L^* galaxies the fraction of
baryons in the circumgalactic medium (CGM) is inversely correlated with the
mass of their central supermassive black holes (BHs) in the EAGLE hydrodynamic
simulation. The interpretation is that, over time, a more massive BH has
provided more energy to transport baryons beyond the virial radius, which
additionally reduces gas accretion and star formation. We continue this
research by focusing on the relationship between the 1) BH masses, 2) physical
and observational properties of the CGM, and 3) galaxy colours for Milky
Way-mass systems. The ratio of the cumulative BH feedback energy over the
gaseous halo binding energy is a strong predictor of the CGM gas content, with
BHs injecting >~10x the binding energy resulting in gas-poor haloes. Observable
tracers of the CGM, including CIV, OVI, and HI absorption line measurements,
are found to be effective tracers of the total z~0 CGM halo mass. We use
high-cadence simulation outputs to demonstrate that BH feedback pushes baryons
beyond the virial radius within 100 Myr timescales, but that CGM metal tracers
take longer (0.5-2.5 Gyr) to respond. Secular evolution of galaxies results in
blue, star-forming or red, passive populations depending on the cumulative
feedback from BHs. The reddest quartile of galaxies with M_*=10^{10.2-10.7}
M_solar (median u-r = 2.28) has a CGM mass that is 2.5x lower than the bluest
quartile (u-r=1.59). We propose strategies for observing the predicted lower
CGM column densities and covering fractions around galaxies hosting more
massive BHs using the Cosmic Origins Spectrograph on Hubble.
