I want to know why it is called blowing raspberries, where does it derive from & why do we still call it that?

"Blowing raspberries" comes from the slang meaning of "raspberry", Daniel.

Dictionary.com says:

4. Slang. A derisive or contemptuous sound made by vibrating the extended tongue and the lips while exhaling.

Not sure why the raspberry got singled out for this indelicacy. [They don't deserve it.]

Dictionary.com proposes this explanation:

Obsolete raspis, raspberry + berry. Sense 4, possibly short for raspberry tart rhyming slang for fart.