Its difficult to say "this one works, this one is less good because its going to depend so much on the quality of the blackberries and how "wet" the juice is, also how sweet you like your liqueur. I have had great success with cranberries and with cherries using a very simple method. Get a large (four pints ish) jar, clean it well even if its unused, put it through the dishwasher if you have one, and a bottle of decent quality neutral flavoured vodka. At least 3/4 fill it with fruit uncompacted. Tip in sugar until all the little gaps in the fruit are full. If the berries are sharp, you might like to add a little more. Fill to the top with vodka. Put the lid on tightly and leave in a cool dark place, not the fridge. Shake daily for the first couple of weeks if you remember. Do shake it though. once the sugar has dissolved, or about two months in, have a taste. It will taste a bit rough but you can judge if it needs more sugar. leave for another three months ish, shaking occasionally, taste again. At six months strain out the solids and put the liquid through a a wine filter or a coffee filter to polish it. Check sweetness and adjust. Put away in a scrupulously clean jar or bottle. leave for another three months, six is better. i have had two seconds at the local show with liqueurs made by this method.