Spread the fresh blackberries in a single layer either in a 9x13 baking dish or a cookie sheet.
Bake in a 350° pre-heated oven for 30 minutes-the fruit will be really hot and puffy but still holding its shape
Remove from the oven and immediately stir the sugar and lemon juice into the hot berries, stir to combine, mushing up the fruit and the sugar dissolves
With seeds and fruit pulp:
If you don't mind the seeds and the fruit pulp now is the time to divide it into jars and put in the refrigerator to set up
Seedless:
If you prefer seedless now is the time to run the mixture through a food mill or mesh strainer - discard the seeds and fruit pulp
Place the jam in jars and set in the refrigerator to set up.
Store in the refrigerator
Notes
This jam will not be as set-up as store-bought or a recipe that uses pectin but is still spreadable but not runny.
Nutrition values are estimates, for exact values consult a nutritionist.