Make a smoothie of blueberries and ice. Lots of ice. You can use some orange juice to 'soften' the mixture, make it less chunky and a little more liquid. The blueberries will help with the constipation.
It may be that your child wants new textures. You could freeze this in ice cube trays and put it in one of those 'fruit strainer' bags they can chew on. I've included a link so you know what I'm talking about:
http://www.target.com/p/munchkin-fresh-food-feeder-2pk/-/...
Is your child teething, perhaps? This will feel good and give pressure while allowing the baby to have something which will encourage bowel movements and the ice will get the water in there without the actual act of drinking. Also, mix in more water with the baby food you are feeding. Back off on any rice cereal, too, or empty 'rice' carbs, which can be constipating. (Including those 'num num' things or grain-based treats). Just fruits, veggies, water and formula.
IF you decide to do prunes, be sure to do baby food ones. They need water added to them, to rehydrate them, or they will absorb that water from baby's colon, continuing the problem.