Slice off just enough from the bottom of the watermelon so that it can sit flat and stably on the serving plate.
Use a pen to draw on the face.
Very carefully, use a paring knife to cut out the face. The face should be large enough that you can easily use a spoon inside the watermelon.
Use a spoon to scoop out the watermelon around the mouth area. You want to keep some watermelon around the eye area so that it looks red when you look in the eye holes (instead of seeing just white watermelon rind).
Place the watermelon in a large mixing bowl and combine with all the other ingredients, except the blackberries.
Mix until combined.
Place the watermelon on a large serving plate.
Place some of the fruit salad in the watermelon mouth and some of it on the plate.
Spread the blackberries evenly around the fruit cup (you add them separately so that they don’t get smashed).
If you have leftovers, remove them from the watermelon, place in an airtighter container, and refrigerate for up to 3 days.