The adjective "dark" in Latin is obscurus (-a, -um) or tenebrosus (-a, -um). "Dark-colored" is ater (-tra, -trum).

"The dark" (i.e., darkness) is tenebrae (-arum, f.).

Tenebrae, -arum is the Latin word for "darkness" or "the dark". It appears only in the plural. The adjective "dark" is tenebrosus, -a, -um.
I will list the Latin words meaning "dark" (as a noun):

• Opscurum
• Obscurum
• Obscurit
• Opscuritas

Whichever these you choose, they should mean the noun "dark". If you go on Google Translate, there are also adjectives meaning "dark" as well. These are just the noun ones.