It depends ...
...
Blackberries (one 144g serving) contain 14g of Carbohydrate (with some being from fiber) and so it is considered a Keto food, if you count it as part of the 25g limit of carbs you'll have per day.
But here's the kicker; some of that carbohydrate (7.0g) is fiber which means that a whole serving has less than 7 grams of carbs in it.
As long as your total carb count at the end of the day is under 25-40g then eating any carb is fine, and blackberries are a healthy choice if you do eat them.
Blackberries have almost no fat and a small amount of protien which means they are not the typical, fat-heavy, low carb Keto food but they are still able to fit into the Keto diet if you choose.
Controlling insulin is key when it comes to the Keto diet and adding in blackberries is not a danger to your staying in ketosis unless you eat too many.
Eat and be healthy.
...