Blackberries are…black…berries. This makes them fruits.
They’re a very dark purplish-red color that makes them look black. They’re not brightly colored, but their juice has a very strong color. It’s just dark when the blackberry’s all filled up with it. This is characteristic of fruits.
They’re also very sweet, if sometimes a little bitter. They’re always eaten raw or in a pie or cobbler. They’re also botanically fruits, which is an important qualification. All together, they’re obviously fruits.
Interestingly, they’re not botanically berries though. They’re actually an aggregate fruit where each individual “drupelet” would actually be a separate fruit. These fruits would be drupes though, still not botanical berries. Yet, cucumbers are botanical berries. This evidences how you shouldn’t care about botanical definitions.