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".
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).