Darker doesn't always mean blacker. Symbolically, a blood moon is "darker" (as in "ominous" and "eerie") than a new moon. The red color has many meanings, ranging from passion to wrath. Even after science emerged to explain such phenomena (the red color being just the longest wavelength part of visible electromagnetic spectra, the blood moon being just a combination of physical and astrophysical factors such as Rayleigh scattering and planetary alignment, etc), the blood moon still gets a "bad omen" vibe nowadays, a vibe that's absolutely not present during new moons (it's worth mentioning that they happen once or twice every month, differently from a blood moon which is a somewhat-rare event).
The joke in my part of the world used to be "a black cat in a coal cellar at midnight". That this is also a cat makes me think that the artist might be familiar with that idiom.
Mine was "darker than a black cat in a coal mine at night" but I think it's just easier for hicks with an accent to say. Far less racist than the other ways they would say "dark".
The darkest thing is the universe, in about 10^10¹²⁰ years (or seconds, or stellar lifespans, it's all the same at this time scale), after every star has died, every black hole dissipated, and every material object quantum tunneled out of existence, when energy is as dispersed as it can possibly be
And then the concept of "and then" ceases have meaning, when the ratio between the time when there are things and the time when there are no things approaches 0