I didn't have much time to pick the blackberries yesterday so when I weighed them today there was just under 3lbs. That's not really enough for a big batch but would do for a start.
The recipe I follow is incredibly simple, if fact the hardest part is sterilising the jars and washing up afterwards.
First, wash the blackberries then put into a large pan with half a pint of water. Slowly bring to the boil and mash down with a potato masher to extract as much juice as possible. Once it's all gooey you need to remove the seeds and the easiest way to do this is through a jelly bag. You can buy these at any good kitchen shop or online from Amazon HERE. Alternatively you could use a piece of muslin and a wire strainer/colander.
When it's stopped dripping (I left it for about 2 hours while we went shopping) you'll be left with some nice clear juice and a bag of mushy seeds which you throw away.
Before you start making the jam, put 2 saucers or small plates in the freezer, you'll need these later to test if the jam's ready for potting. Wash your jars and lids in hot soapy water and rinse well, no need to dry. Put the jars on a baking tray and into a cold oven. Turn oven on to Gas 2 and leave until jam is ready to pot.
Measure the juice in a jug (and make a note of how much you have) before pouring into a large pan. I've still got my old Maslin pan but you could use a pressure cooker pan or a large saucepan.
Add granulated sugar at the ratio of 90g sugar to 100ml juice. I had 850ml juice so added 765g sugar.
Put the pan over a low heat and stir till all the sugar has dissolved. I've got a long handled wooden spoon that I keep just for this job.
Once the sugar has dissolved, add the juice of one lemon and turn the heat up to bring it to a rolling boil. DO NOT STIR as you will end up with the sugar crystallising out on the sides of the pan and your jelly will be "gritty". You can use a thermometer if you have one, or leave it to boil for 15 mins before doing a test to see if it's ready to pot.
To test for a "set" you need one of the saucers from the freezer. Pour a teaspoonful of jam onto the saucer and leave for 1 minute to cool. Then push a clean finger through the jam. If it wrinkles as you push it, it's ready. If not, boil for another 5 mins and try again.
Once it's ready, remove the pan from the heat and pour the jam into hot jars. Seal the top with a wax disc and cellophane wrapper. I also save the jar's lids and re-use them as well as the cellophane. If you don't seal the jars while the jam's still hot, you must wait until it's stone cold to do it. Sealing lukewarm jam will result in it going mouldy. I don't know why, it just does.
My 3lbs of blackberries gave me 3 smallish jars plus a bit left over which we'll have tomorrow.