"He loves dark chocolate."
Translation:Il adore le chocolat noir.
After verbs that dictate likes and dislikes (i.e. aimer, adorer, detester, etc.), one always uses "le/la/les". Why? Because when you say you like something, you don't only like some of that thing. You like all of it. So, for this example, "Il adore le chocolat noir" means that he likes ALL dark chocolate, while "Il adore du chocolate noir" would mean that he only like some unspecified amount of it.
"le chocolat noir" = "dark chocolate" (the category of dark chocolate / dark chocolate in general). "Du chocolat noir" --> "De le chocolat noir" = some dark cholocate. So by exampe: "Il mange du chocolat noir" = "He is eating some dark chocolate" "Il mange le barre de chocolat noir" = "He is eating the dark chocolate bar" (a specific item). Now, unless there was prior reference ("There were two chocolates left." - in English this might refer to single items from a chocolate box), "Il mange le chocolat noir" would not make sense because he cannot eat an abstract category.
In french, most adjectives go after the noun. Colours follow this rule.
So, you'd say "le chocolat noir", "la jupe rouge", "les citrons jaunes".
(But not all adjectives go before. Some, like bonne and grande, go before the noun. It's just a matter of memorization which does which.)