Abstract Davies et al. (2019b) 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 (MBH), 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 significantly higher than the binding energy resulting in gas-poor haloes. Observable tracers of the CGM, including $\rm{C\,{\small IV}}$, $\rm{O\,{\small VI}}$, and ${\rm{H\,{\small I}}}$ 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* = 1010.2 − 10.7 M⊙ (median u − r = 2.28) has a CGM mass that is 2.5 × lower than the bluest quartile (u − r = 1.59). We propose observing strategies to indirectly ascertain fCGM via metal lines around galaxies with measured MBH. We predict statistically detectable declines in $\rm{C\,{\small IV}}$ and $\rm{O\,{\small VI}}$ covering fractions with increasing MBH for central galaxies with M* = 1010.2 − 10.7M⊙.