Preheat oven to 350. Grease & flour a 9X13 inch pan.
If using frozen blueberries, thaw out first. Divide berries in half. Mash 1/2,& save rest, unmashed.
Mix sugar & cornstarch. Add to mashed berries. Cook 5 or 6 minutes on medium heat until thick & clear.
Add lemon juice and unmashed berries. Mix & cool.
In large bowl, mix chopped pecans & cake mix.
Melt butter, pour into cake mix & pecans. Mix until all is moistened. Into greased pan, pour undrained pineapple to make an even layer.
Spread berry mixture over pineapple. Do not stir.
Using hands, sprinkle cake mix mixture over berry mixture, working out lumps in cake mix. Spread evenly.Do not stir.