The Freezer Test for Jams:  Jams can be tricky at times, but Blackberry Jam can give you a run for your money!  It typically takes the longest to set properly. It can take several weeks to thicken up in the jar, or it may not ever thicken with time. Typically slow boiling helps the thickening process. Be sure to boil your jam until it passes the freezer test.  It is a great indicator of how thick the jam will set. Basically, place a thumb-size drop of jam on a plate and stick it in the freezer for 30 seconds. After removing the plate, touch the jam.  When you touch the jam it should be a slightly firm, almost wrinkle a bit when touched.  If it merely smears, it is not ready – if it wrinkles, it should thicken up just fine and is ready to be placed in jars for processing.  Wish I had a quicker fix – just boil and test is my best advice.  It happens to me with my Jalapeño Jelly.  Don’t get discouraged!!!