BlackBerry, the brand name for the communication device is a proper noun. Proper nouns, when plural, take an S even if they finish in Y.

Blackberry, when used to name the fruit, is a common noun. Nouns that finish in Y finish in IES when in the plural form, unless the Y is preceeded by a vowel.

The easest way to visualize this is to imagine having two people in your family called Wendy. When you write this down, you'd write, "There are two Wendys in our family" not "two Wendies"

It only becomes confusing when companies use common nouns as brand names, turning them into proper nouns. We know the word as a common noun and want to treat it that way and the correct way of pluralizing it grates at us.