Blackberry is a compound sign. First, sign black, then sign berry by holding out the index finger of your non-dominant hand, and finally holding the index's tip with all the fingers and thumb of the dominant hand closed in on it, twisting back and forth at the wrist.
Teach your child the blackberry sign when eating or having it as a snack.
Use the blackberry flash card along with the apple and banana flash cards. Grouping fruits will equip your baby to understand categories of favorite foods. As your baby gets more advanced, you can introduce color and use black and red, and brown and green as color descriptors for fruits and food items.