Skin off the onions and garlic, quarter the onions, wash the chillis and cut of the stalks.
Blend the onions, garlic, and chillis.
Gently fry the mixture in the vegetable oil over medium to low heat for 20 to 30 min until the juices of the onions have evaporated.
Blend the canned tomatoes and add them to the onion chilli mix. Add the vinegar.
Mash the blackberries and add them, plus the blueberry juice, paprika, and salt.
Bring the sauce to boil, then let it simmer over medium to low heat for about 1h and stir regularly. It should have a smooth, not too thick and not too thin consistency, similar to ketchup.
Boil a large pan of water. Carefully submerge your jars or bottles into the boiling water, including the lids. Let them simmer in the boiling water for at least 1 minute.
Lay a clean kitchen towel on your worktop.
Sterilise everything that will come in contact with the chilli sauce or jars in the boiling water, including funnels and kitchen tongues to remove the jars or bottles (my funnel is made of plastic, so I only held it in the boiling water for a few seconds).
Remove the jars or bottles with kitchen tongues and put them bottom facing down on the kitchen towel. Remove the lids and lay them top facing down on the kitchen towel.
Fill the jars or bottles with your sauce leaving some space for air, not overfilling them.
Close the lids firmly and turn the jars upside down. Once they’re cooled down turn them around again and store them in a dark, cool place.