1 1/2quartsvanilla frozen yogurtthawed at room temperature for about 20 minutes (you want it to be soft enough to spread easily)
1pintblueberry ice creamgelato or sorbet, scooped into balls using a 1 tablespoon-sized melon baller
1 1/2cupfrozen blueberries
Line a loaf pan with plastic wrap, using enough that there is an overhang for easy release later on. Set aside.
In a large bowl, mix all of the ingredients thoroughly using a heavy wooden spoon. Pour the mixture into the prepared loaf pan and smooth out the top. Cover with plastic wrap and foil and place in the freezer for at least 4 hours or up to a week.
Once completely frozen solid, remove the terrine from the freezer. Release it from the loaf pan using the plastic wrap overhang, and flip it upside down onto a cutting board. Using a very sharp knife that has been dipped in hot water, cut 1 inch thick slices, dipping the knife in hot water before each slice.
You may prepare the terrine in advance, as it may stay in the freezer for about a week.