We are at risk of stating the obvious with the following statement, but for the avoidance of all doubt, nobody has ever actually eaten a blue raspberry. Back in the day, some marketing guru in a trilby invented the colour to stop people confusing one ice lolly with another, and the flavour got made up to match.
But who cares about semantics. The only thing that matters is that it works, largely because it is sweet without being sickly, and there is enough tartness underneath to stop you getting bored too quickly.