The blackberry is a fruit, produced by the Blackberry bush.

You obtain it by talking to Seedos for the seed (you must be Gardener Level or higher) and getting lucky, or purchasing the seed at Costolot's store (Gardener Level 9 or higher). The fruit is also available at Costolot's store (at Gardener Level 9 or higher).

Gardener Level 9
Base Value (seed) 55 coins
Base value (fruit) 100 coins
Base Value (tree) 140 coins
Selling Price (seed) 50 coins (Costolot's)
Selling Price (fruit) 20 coins (Costolot's)
Fertilizer Color Purple
Surface Requirements Plant in grass, soil, or long grass