1. Wash and thoroughly crush blueberries, one layer at a time.
2. Measure 4 cups into a 6 or 8-quart saucepan.
3. (do not use a smaller saucepan or fruit will not set).
4. Stiri in juice concentrate.
5. Stir fruit pectin gradually into fruit mixture in saucepan.
6. Add butter.
7. Bring mixture to full rolling boil on high heat, stirring constantly.
8. Boil exactly 15 minutes, stirring constantly (it is important to stir constantly for 15 minutes to get a good set).
9. Remove from heat.
10. Skimi off any foam with metal spoon.
11. Ladle quickly into hot sterilized jars, filling to within 1/8" of tops.
12. Wipe jar rims and threads with a clean, damp cloth.
13. Cover with two-piece lids.
14. Screw bands tightly.
15. Invert jars for 5 minutes and then turn upright.
16. After jars are cool, check seals (or use water bath method, processing pints 5 minutes after water returns to a boil).
17. Makes about 5 cups or about 2 ½ pints.
18. Serving suggestions: serve with homemade muffins, or spoon over lemon sherbet.