Combine the yogurt, milk and 2 cups raspberries in a blender container.
Puree on medium speed.
Transfer to a bowl and chill.
Slice the tops and bottoms off the pineapples.
With a sharp knife, remove the outer peel and the inner core of each.
Slice the flesh crosswise into 1" thick pieces.
Halve the peaches and remove the pits.
Prepare a grill and place the pineapples and peaches directly on the rack.
Grill over medium-high heat for about 4 minutes per side, or until golden brown.
Use a metal spatula to transfer the fruit to dessert plates.
Sprinkle with the blueberries and remaining 2 cups raspberries.
Top with the sauce.