In this phrase, someone's being compared to a "little ray of pitch black." Imagine it like this: normally, a ray of sunshine brings light and warmth, but this is the opposite. It means the person seems to bring darkness or negativity wherever they go, like a gloomy cloud in a sunny sky.
It's like saying, "Aren't you a bit of a mood spoiler?" or "You seem to bring bad vibes with you." It's a playful way to tease someone when they're not in a good mood or seem to be spreading negativity. So, it's not really mean, just a light-hearted jab to make them smile.
Overall, this quote is a creative way to say that someone's not being very cheerful or positive in the moment. It's like comparing them to a tiny patch of darkness in a bright world.